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Old 01-19-2015, 08:43 PM   #1
mikeheather
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1994 SL2
Default 94 SL2 Idle RPM Drop and Periodic Miss?

Hello:

I have a 94 SL2 that I received for pretty much free. I've gone through and pretty well fixed it up; however, I still have one problem left. Here are symptoms:
  • When idling, RPM will drop from 800 to about 200. Sometimes it will die sometimes it won't. The symptom is there pretty much if the engine is cold or warm, but more common when warm. Seems to be getting worse over time. I would almost now classify it as a rough idle because the idling does not hold at 800 very well.
  • When driving at speed, sometimes there is the feeling of a miss (not sure how to best describe it).

As indicated, I have things pretty well fixed up. Here is a list of related items that have been replaced:
  • Engine cooling temperature sensor and connector
  • Transmission temperature sensor and connector
  • Air intake temperature sensor
  • Transmission valve body pan gasket
  • Idle Air Control Valve
  • EGR Valve & Gasket
  • Thermostat
  • Spark plugs (NGK Standard Copper) and wires
  • PCV Valve
  • Water pump
  • Valve cover gasket
  • Fuel pump
  • Crank position sensor
  • Ignition coils
  • Ignition control module
  • O2 sensor
  • Cleaned the throttle body

I have gone through the forums and haven't much that I haven't already done.

Cheers,

Mike

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Old 01-19-2015, 11:08 PM   #2
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Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

Mileage? Fuel pressure?

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Old 01-20-2015, 09:07 AM   #3
mikeheather
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1994 SL2
Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

Quote:
Originally Posted by fdryer View Post
Mileage? Fuel pressure?
The odometer stopped working around 137K. I have no idea how many miles since then.

Two things I haven't done is:
  • Fuel pressure
  • Engine compression

One other symptom I left out is that it takes extended number of cranks for warm starts. Cold starts, it fires up immediately.

Cheers,

Mike

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Old 01-20-2015, 09:41 AM   #4
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1995 SL1
Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

The compression test and fuel pressure test might be the next course of action. Being you've replaced a lot already, hard to tell what could be left.

You might want to also check out the fuel pressure regulator if it's original. Usually the sniff test is a start, then you can test function if/when you do the pressure tests.

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Old 01-21-2015, 09:11 AM   #5
mikeheather
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1994 SL2
Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

Bit more of an update. While I was driving the car around the block to warm up the engine for the compression test, the engine light came on. Code is 45 - O2 sensor indicates rich condition. Now I did have this prior to replacing the O2 sensor, so it appears that the O2 sensor is just the messenger of something else. Thoughts?

Also, I went ahead and check the compression numbers: 170, 130, 155, 130. Thoughts on that?

Cheers,

Mike

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Old 01-21-2015, 09:19 AM   #6
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1998 SC2
Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

Without explaining exactly how you got those compression numbers they have minimal value, see below.

The rich condition could be a leaking exhaust system, see below.

Exhaust leak check.
--Locate a low pressure high volume air source, reversible vacuum cleaner, large fan, leaf blower or what ever you can find.
--Loosely couple to cold exhaust at the tail pipe, or direct fan at tail pipe.
--Spray every inch of the exhaust between the head and CAT inlet with a mixture of 2 or 3 drops of dish soap and water in a spray bottle.
--Pay close attention to the lower flange, lower support clamp, and flex connector.
--The flex connector is under the protective braid so it requires quite a bit of solution to show any leaks.

Compression testing
Crank the engine until the gauge quits increasing and compare both the final pressure and number of compression strokes to reach the max pressure, cylinder to cylinder. Any other method has no value for comparison in a forum setting. You can do this test either cold or hot or warm just mention it when stating results. For example if you find all cylinders produce a final reading of 200psi but the compression cycles are as follows: 1-8, 2-6, 3-8, 4-14 then it should be obvious that cylinder #4 has a serious problem with leakage past either the valves or compression rings.

To prevent washing the cylinders down with fuel remove the PCM B fuse to shut off the fuel pump, injectors, and ignition before starting the test. Remove all spark plugs and block the throttle open. Use a low charge rate battery charger to fully charge the battery before conducting the test.

FSM states the MINIMUM number of compression cycles is 10 and a Fully charged battery is required to obtain the full 250RPM cranking speed. The FSM also does not mention counting the compression cycles and recording them. This is because the high tech recording tester counts them for you and even a blind mechanic can see the differences on the chart. As most of us are doing this with a simple pressure gauge you have to count them yourself.

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Old 01-21-2015, 09:27 AM   #7
mikeheather
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1994 SL2
Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

On the compression test I followed what was indicated in the Richpin video.
  • Warmed the engine
  • Removed the fuel pump fuse and let the car die
  • Clamped open the throttle-body air intake
  • Unplugged the ICM
  • Removed all spark plugs
  • With the compress gauge attached, I would crank until the needle would no longer go any higher. This was upwards of 20 cranks.
Hope this helps to that regard.

Cheers,

Mike

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Old 01-21-2015, 09:42 AM   #8
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1998 SC2
Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

If you overtighten the tester into the engine it will leak and give low readings. The numbers you have posted would indicate that the engine should probably not even start let alone run. The tester is screwed into the head until the o-ring just contacts the head, do not squash. It will be much easier to just pull the PCM B fuse and block the throttle open. Be absolutely sure the battery is actually fully charged as well.

I have no idea exactly what went wrong but if those numbers are valid you need a new engine.

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Old 01-21-2015, 10:42 AM   #9
mikeheather
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1994 SL2
Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

Quote:
Originally Posted by OldNuc View Post
If you overtighten the tester into the engine it will leak and give low readings. The numbers you have posted would indicate that the engine should probably not even start let alone run. The tester is screwed into the head until the o-ring just contacts the head, do not squash. It will be much easier to just pull the PCM B fuse and block the throttle open. Be absolutely sure the battery is actually fully charged as well.

I have no idea exactly what went wrong but if those numbers are valid you need a new engine.
Okay, so let me put the compression activity aside for the moment. Would an exhaust leak explain the symptoms I'm experiencing?

Cheers,

Mike

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Old 01-21-2015, 06:17 PM   #10
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1998 SC2
Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

It can be part of the problem. It is a free item to test and there are several gaskets in that exhaust system that may easily fail.

I would go to a parts store and have the battery and alternator load tested as low voltage will do this. The ignition system is incredibly sensitive to minor low voltage conditions.

Other items in no specific order are:
ECTS
Thermostat
Dirty Throttle body
shot plugs or incorrect type
defective plug wires
failing ICM
failing TPS

Thee are some more possibilities but they are a much lower probablility.

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Old 01-23-2015, 10:29 PM   #11
mikeheather
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Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

I'm getting closer. Took the car to the parts store and had the load test done on the battery and alternator. Battery is good. The output on the alternator failed sometimes and passed sometimes. Seems like the failure lined up when the engine/idle was struggling. So the guy told me to go through and check all the grounds, before ripping out the alternator.

Cheers,

Mike

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Old 01-23-2015, 10:58 PM   #12
mikeheather
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Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

Quote:
Originally Posted by mikeheather View Post
So the guy told me to go through and check all the grounds, before ripping out the alternator.
So any particular ground connections I should be looking for? The obvious ones seem good (battery to engine and chassis.

Cheers,

Mike

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Old 01-23-2015, 11:30 PM   #13
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Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

Power to the alternator is thru a fusible link wire attached between the alternator post and starter solenoid. The fusible link wire is one of two wires on the starter solenoid - the other wire is from the battery. Be sure this fusible link wire isn't frayed on both ends. Tug each connection and see if the wire pulls apart. It shouldn't. You can measure battery voltage at the alternator. If voltage is good there, alternator ground is its case bolted to the engine block. Engine ground must be clean and free of corrosion to the chassis stud which should be clean and free of corrosion. If grounds and power connections are good, replace the alternator.

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Old 01-27-2015, 09:43 PM   #14
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Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

Problem solved!!

The solution was the fuel pressure regulator. Once that was replace, the car runs like a champ. Can sit at a stop light forever and the RPMs sit at about 700 and never budge. Can take the car on the tollway and cruise at 70+ MPH and no drops in power.

Seems like I have replaced about everything on the car. Beside what was listed above, the car has all new suspension and front-end as well. Car handles great. Now my two teenagers can drive a reliable car all around town. Only thing I have to worry about is them wrecking it

Cheers,

Mike

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Old 01-28-2015, 07:06 AM   #15
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Default Re: 94 SL2 Idle RPM Drop and Periodic Miss?

On the plus side, your teenagers should now have a car that has very few sensors and such that aren't new. Make sure they don't tear it up, and make them check the oil a LOT.

If I let my daughter start using the SL1, an oil check log will be a "mandatory or you lose the keys" thing.

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