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Old 04-26-2018, 03:00 PM   #1
Dunkinboom
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Default MAP sensor question

My 97 SC2 is a circle track car. Ported head, Gude race cams, header, Storm throttle body, and Wiseco 13:1 pistons. My question is can I run it with out a Map sensor? The car runs good other than at idle unless the sensor is unplugged. I understand the sensor interprets how much load the engine is at, but it stalls and hunts for a happy place to stay running. I can turn up the idle but then runs too high. Checked for vacuum leaks and none found. And yes I have tried another Map sensor. Any suggestions?

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Old 04-26-2018, 03:33 PM   #2
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Default Re: MAP sensor question

You might have the best of two methods if this idea works for you - wire a switch inline to one wire on the sensor. Switch it on or off. This should give you the best of both situations, off when needed and on when needed. As a load sensor, its only for naturally aspirated engines that sucks in air (no turbo or supercharger). Since n/a engines create a vacuum when the engine runs, the 1-bar/atmosphere map sensor measures from near absolute vacuum (29.99in hg) to sea level pressure (14.7psi) hence the 1-bar rating. Turbo and superchargers generate positive pressure above ambient atmospheric pressure so 2 or 3 bar map sensors are needed with factory programming ecm/pcm to detect the change from vacuum to positive pressures, allowing the ecm/pcm to alter the fuel mixtures without creating an extreme lean condition when positive air pressures means more air than fuel results in a blown engine. And if you're thinking of throwing in a 2/3 bar map sensor, you'll screw up the ecm/pcm when its set for 1 bar sensors.

If you have the time, there's something wrong with your setup that's confusing the pcm, creating the problem you're experiencing. If the on/off switch works then you can leave it as is and incorporate the routine of switching the map sensor on/off into your routine for track racing.

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Old 04-27-2018, 07:45 AM   #3
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Default Re: MAP sensor question

Thanks for the reply and info. Will it run to lean leaving the sensor unplugged?

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Old 04-27-2018, 08:20 AM   #4
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Default Re: MAP sensor question

Quote:
Originally Posted by Dunkinboom View Post
My 97 SC2 is a circle track car. Ported head, Gude race cams, header, Storm throttle body, and Wiseco 13:1 pistons. My question is can I run it with out a Map sensor? The car runs good other than at idle unless the sensor is unplugged. I understand the sensor interprets how much load the engine is at, but it stalls and hunts for a happy place to stay running. I can turn up the idle but then runs too high. Checked for vacuum leaks and none found. And yes I have tried another Map sensor. Any suggestions?
Is this engine naturally aspirated or forced induction?

Is the PCM still stock?

What is the lift, duration, and lobe centerline profiles of the camshafts?

Unplugging the MAP sensor will default the PCM to fuel maps with reduced power output, because it will cause a DTC.

In other words leave the sensor plugged in.

You can try a 2-BAR MAP, but this will run the engine WAY TOO rich, and the O2 sensor will set DTCs trying to lean it out with the short term fuel trims.

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Old 04-27-2018, 08:50 AM   #5
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Default Re: MAP sensor question

Engine is just factory injection.

Intake lift .368 Duration 236 degrees

Exhaust lift .344 Duration 238 degrees

Stock pcm

10-12 in vac @ idle

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Old 04-27-2018, 10:18 AM   #6
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Default Re: MAP sensor question

Dunkinboom, here's a good site to learn how map sensors are used; http://www.aa1car.com/library/map_sensors.htm. I'm no expert on EFI systems but know enough to understand there's always a reason for engineers to have parts on our engines. Someone miscalculated and caused deaths when a newly designed walkway was set in place over a six or eight lane road in Florida. It collapsed. Whether the roads should have been closed that day until it was deemed absolutely safe to allow traffic underneath it before it was fully erected remains unanswered until the NHTSA determines where fault lies. NHTSA is very good at finding why commercial airlines crash and preliminary info says the bridge span was already weakened when hoisted into place over the road way. EFI engines relies on every sensor feeding signals to the engine computer to make the right fuel mixtures. Disconnecting the map sensor simply means the engine doesn't have a way to know airflow other than throttle position. It's like having one eye closed. You can see but you loose perspective. Two eyes gives a person three dimensional perspective. One eye doesn't. Disconnecting a map sensor cripples the engine computer. A guess might be leaner fuel mixtures. One eye would be the throttle position sensor, the other eye would be the map sensor.

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Old 04-27-2018, 12:05 PM   #7
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Default Re: MAP sensor question

Quote:
Originally Posted by Dunkinboom View Post
Engine is just factory injection.

Intake lift .368 Duration 236 degrees

Exhaust lift .344 Duration 238 degrees

Stock pcm

10-12 in vac @ idle
Ok, and thank you for the vacuum inches @ idle.

Your vacuum is way too low, due to the camshaft duration. You have a very high-rpm duration.

What I recommend is to punch the cover off the adjustment screw for the Geo Storm TB. Underneath it is a set screw that will depress the throttle and increase idle rpm. This will NOT set any DTCs. I would idle your engine between 1800-2400rpm and check vacuum. Vacuum should probably be above 17" Hg @ idle to smooth it out.

That engine build should not be idling at the factory default 750rpms, whatsoever. Volumetric efficiency is way to low, because air velocity and volume are way too low to properly pressurize the cylinder. This is why it is stalling and idling poorly.

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Old 04-27-2018, 01:01 PM   #8
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Default Re: MAP sensor question

Sorry to disagree with you but you're wrong with adjusting the throttle stop screw. This is a mechanical setting with the bypass air port blocked to adjust for the initial idle between 500-600 rpm, performed (I presume) at factory assembly. When the bypass air port is unblocked, the EFI system detects the rpm and automatically adjusts the idle air control valve to allow metered airflow from the bypass air port to raise rpm to electronically controlled idle, 650-850 (warm engine). All idle speed control is electronically adjusted with the iacv, not the throttle stop screw. Cold engine running idle is adjusted the same way using the iacv and engine computer. Adjusting the throttle stop screw may raise idle until the pcm can't adjust idle any longer as the pcm knows idle is incorrect and tries to bring it back to programmed idle speed. In this case the cams running lower vacuum means the map sensor is detecting higher air flow (incorrectly) and richens fuel mixtures. Switching off the map sensor then forces the pcm to rely on throttle position.

You are correct to point out a lower than normal vacuum and is probably the reason for a lumpy idle the EFI system can't deal with. This is a modification from factory stock setup, making this engine a custom setup with problems not accounted for in stock setup using stock pcm programming. Since every muscle car now made uses EFI systems, they're tuned long before hitting the assembly lines so owners can just push the pedal and use premium gas without the hassle of tuning. This track car is no longer stock and needs tuning. Normal idle vacuum should be between 15-20 in. As suggested, if switching the map sensor works then this should work in this unusual situation with an easy fix for a track car.

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Old 04-27-2018, 03:19 PM   #9
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Default Re: MAP sensor question

Dunkinboom, if you're certain disconnecting the map sensor allows a decent idle compared to having it connected, I may have a workaround with the switch mod. As Saturn Night noticed from your engine stats, the lower (relative to zero gauge/atmospheric) vacuum compared to normal (higher) vacuum due to cam timing keeping valves open longer for high rpm scavenging but lumpy idle unless rpm is raised interferes with the programmed stock pcm setup. The pcm interprets this low vacuum as throttle opened as in acceleration so its adding more fuel at actual idle rpm resulting in a rich fuel mixture. Thie extra rich fuel and greater valve opening time increases the lumpy idle. Unfortunately, as a one off mod, you become the beta tester, inheriting any issues when altering stock mechanical configurations the pcm isn't programmed to account for. A work around may be wiring a mini leaf switch onto a bracket mounted to the throttle where opening throttle closes the switch.

The leaf switch can be either normally closed/held open or normally closed/held open. You select how it's used on the bracket so the leaf switch triggers open or closed to activate/deactivate the map sensor when opening throttle. The idea here is to have the map sensor switched off at idle/closed throttle, map sensor switched on when throttle is opened. Test the switch setup first to be sure this works before making a bracket and buying a leaf switch. A simple pair of small gauge wires extending into the interior with any on/off switch temporarily held in your hand becomes a test setup while you coordinate switch operation to pedal. If this temporary setup works, you can move forward to making a custom switch bracket to mount a leaf switch to operate with throttle to mechanically operate the switch.
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Old 04-29-2018, 09:30 AM   #10
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Sad Re: MAP sensor question

Quote:
Originally Posted by fdryer View Post
Sorry to disagree with you but you're wrong with adjusting the throttle stop screw. This is a mechanical setting with the bypass air port blocked to adjust for the initial idle between 500-600 rpm, performed (I presume) at factory assembly. When the bypass air port is unblocked, the EFI system detects the rpm and automatically adjusts the idle air control valve to allow metered airflow from the bypass air port to raise rpm to electronically controlled idle, 650-850 (warm engine). All idle speed control is electronically adjusted with the iacv, not the throttle stop screw. Cold engine running idle is adjusted the same way using the iacv and engine computer. Adjusting the throttle stop screw may raise idle until the pcm can't adjust idle any longer as the pcm knows idle is incorrect and tries to bring it back to programmed idle speed. In this case the cams running lower vacuum means the map sensor is detecting higher air flow (incorrectly) and richens fuel mixtures. Switching off the map sensor then forces the pcm to rely on throttle position.

You are correct to point out a lower than normal vacuum and is probably the reason for a lumpy idle the EFI system can't deal with. This is a modification from factory stock setup, making this engine a custom setup with problems not accounted for in stock setup using stock pcm programming. Since every muscle car now made uses EFI systems, they're tuned long before hitting the assembly lines so owners can just push the pedal and use premium gas without the hassle of tuning. This track car is no longer stock and needs tuning. Normal idle vacuum should be between 15-20 in. As suggested, if switching the map sensor works then this should work in this unusual situation with an easy fix for a track car.
I have a Geo Storm throttle body on my blown engine, sitting in a garage. There IS an adjustment screw for the throttle cam, and it is sealed at the factory with a freeze plug. Adjustment of that screw will cause a voltage signal from the TPS, since the cam to the throttle is physically moved by turing that screw. Once TPS voltage is detected, the IAC system is no longer used by the PCM.

This is why you can hit the throttle just a tad, at idle, and rev your engine without the IAC trying to restrict flow or RPM. As the throttle opens, the IAC actually closes a little.

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Old 04-29-2018, 01:36 PM   #11
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Default Re: MAP sensor question

Opening throttle allows more air, map sensor detects a drop in vacuum (load sensing), throttle position sensor is detected as the pcm adjusts fuel mixture. I see where you're going but there's no explanation in any service manual about your description to adjust throttle. The only adjustment to throttle from FSMs is their reference to factory setting with bypass air port blocked to prevent supplemental air bypassing the throttle plate opening in the bore hole to adjust for 500-600 rpm. The pcm, from programming, knows the idle shouldn't be low so it retracts the iacv in attempts to raise rpm. When the bypass air port is unblocked the rpm jumps up immediately as a result of the supplemental air flow and fully retracted iacv. The pcm sensing higher than normal idle automatically extends the iacv until engine rpm returns back to programmed (warm) idle (650-850). All this occurs in about two seconds from blocking the bypass air port to unblocking it. I haven't found any technical descriptions to correlate adjusting throttle plate opening than what's described. Remember, this car has a non stock cam that changes airflow. Since you pointed out that it's most likely a high lift cam to keep valves open longer for better high rpm breathing, this affects low rpm idling - the lumpy idle unless rpm is raised above 1k or more. The stock pcm can't adjust for this since the mechanical portion (cams) isn't stock. The op mentioned raising idle but it increased too much. This suggests adjusting idle didn't work. There's no harm in experimenting and he has choices to try. This discussion brings up another idea.

As I recall, worn throttle position sensors cause a high idle problem. If the tps were 'misadjusted', maybe idle can come up some, enough to prevent stalling while not too high for xmission grabbing (below torque converter stall speed). Experimenting with the tps might help but that's for the op to decide on whether or not to spend the time on these ideas.

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Old 04-29-2018, 03:48 PM   #12
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Default Re: MAP sensor question

We're not working with the stock TB. He has a Geo Storm 1.6L TB on the manifold, according to his post.

His issue is both lack of air volume and air velocity. Opening the throttle allows for more volume, which also increases average piston speeds(as engine rpms increase). The higher average piston speeds will allow for higher velocity. The volume is needed to the extra fuel sensed by the MAP having high voltage(low vacuum). The velocity is needed for turbulence(which because the camshaft duration and head porting is very low, as the valves are open too long and the port surface area has minimal restriction). The turbulence helps to atomize the fuel better, which allows for better cylinder pressurization and smoothes out the idle.

An engine's vacuum signal is actually directly proportional to its operational efficiency. The very first BMW "fuel economy" gauges, were nothing more than a fancy vacuum gauge hooked up fr the cluster, to the engine.

With his cams, he will be blessed to be able to achieve 17-18" Hg @ around 1,500rpm.

That engine will not see very high idle, unless cruising down the freeway at about 4,500-5,000rpm.

And even then, if he could break the low 20s for vacuum, I would be shocked.

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Old 04-29-2018, 04:51 PM   #13
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Default Re: MAP sensor question

I overlooked using a Geo Storm throttle body but it doesn't change the fact that this engine isn't stock that's making it difficult for the stock pcm to work with. I fully understand your explanations but you may not see how difficult it is for fixed programming in EFI systems to adjust for non stock parts like aggressive cams and a larger throttle body bore with a different throttle position sensor. All we can do is suggest trying things and see which one works.

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Old 04-30-2018, 09:05 AM   #14
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Default Re: MAP sensor question

I really appreciate everyone input on this thing. Here's the update.

BTW I did run these cams in a totally stock motor last year with no problems. Ya the lumpy idle but didn't have to adjust anything-just get in and go.

Possibly the higher compression/porting etc is causing this condition-or a faulty PCM.
Went to the track Saturday. Installed a toggle switch on dash to reinstate the MAP signal. First couple laps car ran great-then when off throttle car would bog throttling out of turn-them clear up. Cracked throttle open little more with idle screw and retried. Car ran great-just needed little more air off throttle coming into turn. Kept MAP turned off during tire warm up, then when it was go time turned MAP on to compensate for load. Ran 5 practices. Came in 3rd in heat and 6th out of 16 cars for feature-not bad on new track.

I'm not sure if this style PCM can be tuned-no since of even trying to have this one tuned if it is bad

For running against chipped 200hp Hondas, the car did great. Little more suspension and she'll be more compeditive.

I do appreciate the help from everybody.

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Old 04-30-2018, 09:58 AM   #15
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Default Re: MAP sensor question

Quote:
Originally Posted by fdryer View Post
I overlooked using a Geo Storm throttle body but it doesn't change the fact that this engine isn't stock that's making it difficult for the stock pcm to work with. I fully understand your explanations but you may not see how difficult it is for fixed programming in EFI systems to adjust for non stock parts like aggressive cams and a larger throttle body bore with a different throttle position sensor. All we can do is suggest trying things and see which one works.
Oh, it's ok that you missed the Geo Storm TB part. We all make mistakes. I understand the limitations of stock fuel maps in a PCM, my friend.

The brainbox is the biggest limitation to these engines, with the exhaust ports being the second.

It's all good.

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Old 04-30-2018, 10:01 AM   #16
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Default Re: MAP sensor question

Quote:
Originally Posted by Dunkinboom View Post
I really appreciate everyone input on this thing. Here's the update.

BTW I did run these cams in a totally stock motor last year with no problems. Ya the lumpy idle but didn't have to adjust anything-just get in and go.

Possibly the higher compression/porting etc is causing this condition-or a faulty PCM.
Went to the track Saturday. Installed a toggle switch on dash to reinstate the MAP signal. First couple laps car ran great-then when off throttle car would bog throttling out of turn-them clear up. Cracked throttle open little more with idle screw and retried. Car ran great-just needed little more air off throttle coming into turn. Kept MAP turned off during tire warm up, then when it was go time turned MAP on to compensate for load. Ran 5 practices. Came in 3rd in heat and 6th out of 16 cars for feature-not bad on new track.

I'm not sure if this style PCM can be tuned-no since of even trying to have this one tuned if it is bad

For running against chipped 200hp Hondas, the car did great. Little more suspension and she'll be more compeditive.

I do appreciate the help from everybody.
If you are allowed to change the PCM, invest in a MegaSquirt controller for your engine, even though it is naturally aspirated. It is fully customizable and comes with its own MAP sensors.

The Saturn PCM can be hacked, but I do not know of anyone that actually does it. All GM 1996-up(OBD-II) systems can be reprogrammed as the PROM is re-writeable(unlike 1982-1995 GM OBD-I systems).

However, you usually need a thorough and comprehensive understanding of the GM Tech-II equipment, and hexadecimal computer code to reprogram them.

As far as suspension, you will want to invest in aftermarket. The SC2 came with the "best" suspension(stock) available during production.

Look for AMR Engineering, here in our forums.

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Old 04-30-2018, 10:13 AM   #17
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Default Re: MAP sensor question

R u talking about on the rear of the TB the round part next to iac? Not the idle screw correct? I wasn't sure if that was it so I didn't want to cripple it for the weekend.

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Old 04-30-2018, 10:21 AM   #18
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Default Re: MAP sensor question

Supposed to run a stock PCM but the Hondas run chips. Supposed to be able to pull out pcm and swap with one they give you if they feel your cheating. But little do they understand mine would need to be reflashed with vin to even start. Not sure how Hondas are set up.
Running stock struts with special rate smaller springs. Just trying to get all the camber I can(no rule).

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Old 04-30-2018, 10:32 AM   #19
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Default Re: MAP sensor question

Another question I have with the TB. TPS is set at .5V @ idle. It only goes to 80% according to scan tool WOT. I tried another one and that's all I can get out of it. Even tried and adjustable one set at .5v and can't get 5v WOT

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Old 04-30-2018, 01:46 PM   #20
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Posts: 40,672
 

2003 L-Series 3.0L Sedan
Default Re: MAP sensor question

Search all you want as you'll never find anything on programming PCMs for the S-series engines. Every person that swears he or she (with some presuming knowledge of coding) can break into PCMs insist they're getting involved and simply disappear without any replies on these forums. Good luck searching as you won't find programming anywhere on these threads or elsewhere. The answer/alternative is always adding a piggy back megasquirt.

Unfortunately, tweaking the tps may be a dead end. Remember, you're dealing with EFI systems and learning as you go if you aren't technically familiar with your Saturn. You're opening something close to Pandora's box once you start meddling or as it's more commonly known, tuning. Unless you understand the limitations of a fixed pcm that can't be programmed as much as hearsay says they can, PCMs for the s- S-series have never been broken into. Honda's, Corvettes, and other engines with tunable engine computers yes but not S-series PCMs.

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