Weird Idling Issues, Worsening Over Time

[saturnloafer]

Hello fellow Saturn Enthusiasts,

I'm hoping ya'll can help me pinpoint what's going on with my manual 1999 SC2.

Some background:

I got the car around 178k miles, now it has a little over 200k. My father helped source me the car and had his mechanic buddy replace the motor mounts and the valve cover gasket. For the first several thousand miles of me owning the car, it ran great, rarely any problems.

However, it started doing this thing where the RPM's would drop at idle, sometimes stalling. I usually "fixed" the issue but revving it good a couple times. Eventually, around 198k miles, it started doing this all the time. I recently took a 500mi trip, so I made sure to "tune it up" right before. This included:

1. Replaced fuel filter
2. Replaced O2 sensor
3. Cleaned throttle body, IAC
4. Replaced spark plugs (these were corroded as hell and IIRC 2 or 3 of them were dripping oil)
5. Oil/filter change
6. A while back, I also replaced the plastic tipped ECTS with a brass one
7. I also ended up replacing the PCV valve, as mine had a giant crack in it.

After doing all of this, everything felt fine and I made the 500mi trip without a hitch. About 2-3 weeks later, the idling issues are back with a vengeance. Not only was it dropping RPMs/nearly stalling, but the engine is running ragged as hell/loping and the RPMs will shoot up to 2-3k, sit there, then go back down. Basically just all over the place. The exhaust is really "puttery" now too.

I'm not sure if my oil dripping spark plugs have something to do with it, if theres a valve cover gasket issue (even though I shouldn't need an new one yet), or if it's vacuum related. I did try removing that large vacuum hose to the brake booster to see if it made a difference, it did not.

I feel like it might be worth mentioning that after I replaced the PCV valve, the only noticeable difference is that when I popped it into neutral, the RPMs would smoothly drop to about 800 before smoothly rebounding to 900-1100 where it normally sits.

Wondering if I should replace the IAC and the TPS or do something with the valve cover gasket. I'm kind of at a loss in terms of where to troubleshoot next, so if anyone has any advice I'll gladly take it. And before anyone asks, yes it burns oil like crazy and the smoke is a greyish/white color lol.

THANK YOU!

[fdryer]

A short list of things to check on:

*compression test should be around 185 psi.

*fuel pressure test should be around 50 psi.

*rap on the catalytic converter. There shouldn't be any 'marbles' rattling within, indicating a disintegrating catcon that may intermittently block exhaust flow.

*disconnect the two wire connector to the coolant sensor and examine it for corrosion. The original factory sensor cracks and leaks coolant into the connector, corroding them and disrupting the low voltage signals to the pcm. Intermittent signals may cause intermittent fuel mixture disruptions that may translate to erratic engine running.

Compression and fuel pressure gauge are free loaners from AutoZone.

You may be witnessing the idle air control valve operating abruptly with erratic engine running. Under normal conditions, the iacv varies air flow from pcm commands from high idle cold engine startups thru warm-up as the coolant sensor feeds continuous temperature signals for the pcm to adjust idle rpm, smoothly. If something abruptly changes rpm, the pcm would alter iacv opening of airflow creating various rpm changes.

A well worn throttle position sensor exhibits high idling at stops. The typical temporary fix is shutting off the engine then restarting with a return to normal idle rpm only to have the tps vary its signals with the pcm raising idle rpm until repeating engine shutdown/restart.

[Waiex191]

I'd try replacing the TPS and the IAC valve. I've had those issues on my '99 as well. Replaced them during the rebuild and no more funky idle.

[toggenburg]

I would also, pull a few plugs out and see if they are loaded with oil.

[ultravioletnk]

Originally Posted by fdryer

compression test should be around 185 psi.

i'm rocking about 100-110psi on janus, here in denver at 5280ft altitude, and it runs great. not the fastest thing on the road but the "low" compression isn't an issue at all. i think the sohc head just needs a valve grind but one of these days i'm gonna put a dohc one on. it's more important that the compression is consistent (within 10psi or so) between cylinders than that it's some specific number. everyone also seems to forget that you'll see lower psi numbers the higher altitude you're at. rings' motor is pretty much mint and i still only see 150-160psi at this altitude.

Originally Posted by fdryer

A well worn throttle position sensor exhibits high idling at stops. The typical temporary fix is shutting off the engine then restarting with a return to normal idle rpm only to have the tps vary its signals with the pcm raising idle rpm until repeating engine shutdown/restart.

i wholeheartedly agree that the tps is the prime suspect. some silicone spray or deoxit sprayed into one of the vent holes (after removal, they're on the side that faces the throttle body) may help, but it probably needs replacement. unfortunately, the aftermarket ones are total crap, but you might get lucky buying one.

[fdryer]

I haven't come across any data to correlate know good compression values taken at sea level then measuring again, say every thousand feet in altitude so its guessing on a standard compression value of a good engine at various altitudes. Add a worn engine and values change. 150-165 psi is good compression for a worn engine. Fuel mileage may be a better indicator when combining high elevation like denver and lower compression. Since air is less dense, the EFI system compensates for it from the map sensor. Overall, I would think fuel economy is lower at high altitude like Denver. Correct me it I'm mistaken

As you may or may not know, density altitude is a factor in reciprocating engines when flying at altitudes in mountainous states. Power decreases in single engine airplanes as some pilots not heeding density altitude calculations fly into mountain sides from ignoring lower power requiring longer time to fly up to altitude to clear mountain tops. Compression is lowered since air density is lower the higher an engine goes up in altitude.

[fetchitfido]

3% compression loss for every 1000ft elevation so 10k feet is about 30% loss making 200psi at sea level means you'll only see about 140psi at 10k feet. 105psi is still low, but is more like 150psi at sea level.

This page goes over it in better detail but the hosting went MIA around 2012 and the CSS apparently wasn't saved in WayBack so it looks mostly blank: http://web.archive.org/web/201008141...om/altiden.htm


As for fixing OP's problem, I'd do some percussive maintenance with a screwdriver on the various sensors on the engine. TPS/IACV/Map Sensor. Not hard like it owes you money but taps (if tap = change, that's the bad part). The 1000rpm idle suggests a vacuum leak, given the age of these now it could be worn out throttle plate bushings in the throttle body. That ought to be high enough it sets a code for "idle higher than commanded", maybe the ECTS isn't working properly and isn't telling the PCM the engine is warm enough to set the code.

[ultravioletnk]

Originally Posted by ultravioletnk

i'm rocking about 100-110psi on janus, here in denver at 5280ft altitude, and it runs great. not the fastest thing on the road but the "low" compression isn't an issue at all. i think the sohc head just needs a valve grind

added note: that motor burns very little oil, i burned about 1.5qt going all the way to tucson, az and back. i think most of that went when i got into a bit of a freeway sprint with someone in a dodge suv coming up through pueblo. the rings aren't suspect but the valves are.

Originally Posted by fdryer

I haven't come across any data to correlate know good compression values taken at sea level then measuring again

there are a fair few charts you can find with a quick internet search, the correction factors do vary but 1-3% loss per 1k ft altitude increase is a good bet.

Originally Posted by fetchitfido

3% compression loss for every 1000ft elevation so 10k feet is about 30% loss making 200psi at sea level means you'll only see about 140psi at 10k feet. 105psi is still low, but is more like 150psi at sea level.

i think 3% might be a bit pessimistic (at least according to the majority of charts i've found) but it's definitely in the ballpark. whatever the case, i think in general on this forum we need to take altitude into account when talking about compression tests. i've seen a lot of posts where people talk like "180psi or it's dead and you should swap the motor" without a second thought for altitude and that's just nonsense.

[Waiex191]

Originally Posted by saturnloafer

After doing all of this, everything felt fine and I made the 500mi trip without a hitch. About 2-3 weeks later, the idling issues are back with a vengeance. Not only was it dropping RPMs/nearly stalling, but the engine is running ragged as hell/loping and the RPMs will shoot up to 2-3k, sit there, then go back down. Basically just all over the place. The exhaust is really "puttery" now too.

I replaced my TPS on Saturday on the '98. Two drives so far and there is a huge difference. Engine runs a lot smoother and no more random 1700 RPM idle. The most alarming thing it used to do was cut out for a part of a second, a couple times per drive. I haven't seen that since the swap.

Worth noting it's nearly impossible to swap the TPS unless you pull the throttle body. Make sure to have a new gasket handy.

I tried a Standard brand TPS this time.

[ultravioletnk]

Originally Posted by Waiex191

I replaced my TPS on Saturday on the '98. Two drives so far and there is a huge difference. Engine runs a lot smoother and no more random 1700 RPM idle. The most alarming thing it used to do was cut out for a part of a second, a couple times per drive. I haven't seen that since the swap.

yep, the tps is usually the culprit with issues like that... been there, done that on janus.

Originally Posted by Waiex191

Worth noting it's nearly impossible to swap the TPS unless you pull the throttle body. Make sure to have a new gasket handy.

what you need is one of these. they're cheap and absolutely indispensable when you need to get a screw out in a location with minimal clearance.

[Waiex191]

what you need is one of these. they're cheap and absolutely indispensable when you need to get a screw out in a location with minimal clearance.

Cool tool, thanks.

[saturnloafer]

Originally Posted by Waiex191

I replaced my TPS on Saturday on the '98. Two drives so far and there is a huge difference. Engine runs a lot smoother and no more random 1700 RPM idle. The most alarming thing it used to do was cut out for a part of a second, a couple times per drive. I haven't seen that since the swap.

Worth noting it's nearly impossible to swap the TPS unless you pull the throttle body. Make sure to have a new gasket handy.

I tried a Standard brand TPS this time.

I swapped the IAC and TPS, does the same ****. I start the car, rpms shoot to 2.3k, then make their way back to around 1000 where the engine runs ragged as hell and sometimes drops to 800-900.

[saturnloafer]

Originally Posted by toggenburg

I would also, pull a few plugs out and see if they are loaded with oil.

I replaced the TPS and IAC, didn't seem to solve anything. If I pull the plugs and they're loaded with oil again, what does this tell me?

[alordofchaos]

Originally Posted by saturnloafer

If I pull the plugs and they're loaded with oil again, what does this tell me?

That there's a problem with the gasket replacement that was done - it may not have been done at all.

If I understand the order / timing right, it was:
1) mechanic buddy replaces valve cover
2) you get ready for a long trip, replace spark plugs and a couple of them were corroded and covered in oil.

I'd inspect the ECTS connector carefully, or just replace it. It's two wires and you already removed it when you replaced the ECTS, wires should be soldered and shrink-wrapped

I'd also double check the fuel filter and O2 sensor - post up the part numbers if you can find them. We see the occasional issue caused by the parts counter guy selling the wrong part.

Any check engine codes?

[DJCharlie]

I've had this issue on my car for the past couple of years. Gonna try to throw parts at it until i finally figure it out

[almoore]

I just cured my erratic idle .The thermostat was bad.The car would not get
hot enough.

[Highmile]

New spark plug wires after you fix the leaking valve cover gasket. also remove plugs and clean the oil off them.

test coils via ohm and then spark check.

If the above check out/fixed/replaced then I'd look at tps and air idle valve. Either could be failing or lazy

Highmile
'95 SL1 762k and counting

[ZepSL2]

Originally Posted by saturnloafer

I replaced the TPS and IAC, didn't seem to solve anything. If I pull the plugs and they're loaded with oil again, what does this tell me?

did you ever figure it out?