Wooddavid88

I have read many forums here and the impressive "Google" and "YouTube". None have been my exact issue. Also talked with a Corvette Club that has helped a lot too, but...

I have an 89 Corvette. Rough Idle, surging when gas pedal is barely pressed when going down street @ 25 to 35 mph and at stop lights. Car seems to stabilize around 600 - 700 when cold up to 200F, which seems to be around 40F above the 160F required for Closed Loop. At that magical number the engine starts pumping, stumbling, and acting funny driving or in park/neutral. Also, when engine is warm, seems to try to drop off when slowing down to a light. RPM will go below 500 then "catch" itself and pop back up.

I have replaced:
Injectors
Sparkplugs, wires, rotor, and, cap
Cats (very clogged),Y pipe and mufflers (rusted out completely)
Oil, oil filter, transmission oil and filter
Gaskets on Plenum and throttle body and manifold.
Changed PCV Valve and Gaskets.
Oil Pressure sensor.
Throttle Position Sensor (And set to exactly .54v)
Cleaned Throttle Body

The car used to completely stall before all these parts were checked, tested, and found faulty before finally retiring them.

Checked for vacuum leaks and finding nothing. No changes in the RPM or stumble.

Strange observations. When engine is warming up around 150F, if I push the car up to 1500rpm, neutral or drive, smoke, almost like burned oil, appears, then leaves quickly from behind the L98. Investigation revealed nothing. Not even oil.

Car has 78k miles total.

After 200F the car sounds like it starts sucking in a bunch of air through the air filter. As if some one has turned on compressed air and blasting it through the MAF.

Also, after resetting the ECM (battery disconnected for around an hour, then plugged back in) did a test drive and WinALDL reports O2 error, but soon went away.

Driving on the highway seems to go smoothly, but with get a service light that comes on for a second after around 20 minutes of driving at 55mph. It then turns off very quickly, but the only saved code is the O2 claiming a Lean condition.

I have checked for vacuum leaks, but am finding none. Nothing seems to stumble it before 200F.

Past the O2 reading I am getting no other errors.

Last strange observation is that removing the hose off of the plenum that goes to the EVP container seems to let the engine stabilize at 600rpm over 200F.

No smoke coming out of exhaust. No smoke coming out around cats or manifolds. No oil on ground.

If anyone has any ideas I am all years. Been after this one for a while.

After cat replace, injector replace, and plugs, the car is driving worlds better than when I got her. Also, 5k miles on the old plugs and they were just brown, nothing strange. Not black, not white, not blue, nothing out of what manuals claim are normal operation.

This is my first OD1 car. I am used to OBDII and cars from the 60/70s, this is kinda a new beast for me.

Sorry for all the text. Just wanted to make it clear what I have observed and the work done to hopefully narrow down a suspect. Any information would be awesome. I have a manual, but, though useful, hasn't completely solved the issue.

Wooddavid88

Also changed: Fuel Pump, Fuel Filter, and sock filter.

Forgot to add those.

I am wondering if it is something electrical. Possibly the IAC (which has been cleaned, but doesn't seem out of range.) the O2 sensor, or another device.

I am getting a proper 40psi at the fuel rails and can hear the pump turn on and a "straw filling" type sound when I turn the key.

Air filter is a K&N that has been cleaned and oiled. The MAF also doesn't throw any codes and has been cleaned with MAF cleaner.

The MAT on the scanner read out also seems to be jumping all over in temp readings.

The MAP hasn't posted any codes.

The cooling fan turns on at 235F just like clock work.

The oil pressure is reading at around 20psi and goes up and down with acceleration. at around 2000 rpm it is roughly at 44 psi.

ex-x-fire

Check the egr system, anytime it flows at idle/low rpm the engine will run rough.

Wooddavid88

I am going to check it today. One of the corvette club members told me I should have replaced it when I did the injectors, but I didn't think it would be bad after only 78k miles. My inexperience may have come back to bite me.

Just a question. Would the MAT cause any symptoms like this with the weird temp readings it is throwing out? Just wondering how large its role would be in the air/fuel trim settings.

By "weird read outs" I mean that it will say that air temperature is 300F then drop the next second to -40F, then back up to 160F then up to 200F then back down to 16F all in about the time of 5 seconds.

billschroeder5842

Yes, you are on to something here. If you engine is "thinking it is ice cold, then over heating" in just a mater of seconds, you will go from rich/lean way to fast for your engine to handle. Plus your other sensors are going to get wonky. Think of when you were a kid playing "red light, green light>"

While you are in there, double check your Coolant Temperature Sensor. I seen the CTS (about $15 and 20 minutes swap) make a world of difference as it is one of the main ways your engine gets its readings, balancing fuel and air for the condition of your engine.

I swapped mine last weekend (for just good measure) as I was cleaning the Throttle body. No big deal.

aklim

You said you replaced the injectors. With what?

All the passages? Removed the top plate, IAC, IAC housing and cleaned all passages? Reassembled with virgin gaskets?

How did you check for vacuum leaks and determine there were none? What was the IAC count when engine has reached a stable operating temp with nothing but the motor on?

It could be the IAC opening up. Mine sounds like a vacuum cleaner.

If air is getting in somehow, it is unmeatered. If it is not accounted for, the ECM might KNOW it is getting 500 grams but it is actually getting 800 which it doesn't know about. That balance of 300 might make the mixture lean hence the code.

aklim

Lets "trust but verify". What is the fuel pressure with the key cycled to "run"? IOW, KOEO. Does it go to 40 and hold even after the key is turned off? Is the pressure holding or dropping fast? After this, take the car for a WOT run. Is the pressure holding? If not, you have a delivery issue. Is the spark good? Are your wires leaking? What about your temperature. How do you know what you have? I would check the coolant temp against an IR thermometer. Scan the sensor and start the car. Does the ECM reading agree with the IR sensor within reason? Does it keep agreeing?

How did you clean the IAC and what do you mean "within range"? At hot idle, it should be about 30 counts with everything but the motor off. If not, you have either adjustment and/or leaking issues.

Do you have a MAP or MAF?

VikingTrad3r

is the coolant temp reported by the ecm via winaldl cotrect?

if your mat is all over, check for severed or crushed wire back by the dizzy. or loose frayed connection.

i wouldnt normally go to sensors first rather a methodical fuel/spark/compression method but that mat is screwed.

Wooddavid88

Wow, thank you for getting with me. I really do appreciate all the help I can get.

From the top:

Injectors were replaced with ACCEL Performance Injectors 22pound. (6 months ago)

Yes, cleaned the IAC and all housing when I took apart the Throttle Body. After that the car started running much better when the gas pedal was pressed while at Idle at lights and in park in the garage. (3 months ago) It was after this cleaning that it sounds like a jet engine in the front of my car when the engine starts warming up in park.

I checked for vacuum leaks with propane. Nothing on the outside of the engine seems to be letting anything on the inside of the engine. Doesn't rule out that something could be letting something pass through a passage on the inside though. The engine was compression tested and found to be, as the mechanic put it "not even broken in yet" and "darn near brand new". I don't have the exact numbers though.

Fuel pressure with key on says and stays at 42psi. Doesn't waver. Also takes a very long time, very long before the psi drops with the key off. We are measuring in a day and a night. By morning the car wants to start, but takes it's sweet time til catching and rolling over. Probably 6 seconds or so. After that start she will quick start, barely a key turn, the rest of the day. No smell of gas or anything in the morning either, so not sure if there is a leaking injector.

A bit of fouled logic, but I lack a thermal scanner. All I can say is that WinALDL agrees with the dash read out, but both run on the temp sensor, so... kinda wouldn't help if the sensor is gone.

I do believe I have a MAF. (Large black tube before the snorkel in front of the Throttle Body, but right behind the air filter housing and radiator?) Though I will say, when putting the plenum back together after the injector swap, I forgot to tighten one of the screws a bit under the plenum and it popped up a MAP error, but right when I tightened the bolt, the Check Engine Light turned off.

WinALDL also claims that I am missing a "CAL PACK", but my reading leads me to believe that the 89 has a different version (PROM?) than what the scanner would be looking for.

The IAC goes up to 160, bounces around 100 to 120 until pedal is pressed where it just drops to 0. Also, when pedal is pressed, large air sucking sound leaves the front of the engine.

All new seals on Throttle Body, Plenum, and Runners from when I put in the Injectors.

Plugs and wires are AC Delco and are brand new (1 week) since install. Also gapped to .35 with a feel-o-gauge.

I checked behind the engine and didn't find any torn or destroyed wiring. Everything looks nice and clean.

Now the car has always claimed a lean condition since I got it, but it was in pretty bad shape when I got it. What led to injector replacement was a "no start when hot condition" and thanks to these forums I Ohm tested and found 3 of the injectors all the way down to 2 when the engine was hot. Also, if I left #5 plugged in at all, the engine wouldn't turn over, just crank.

The new ones, if I unplug any spark wires or injectors, the idle gets way worse, so I assume they are working. I checked the spark wires for spark today with a tester and they are firing consistently.

The TPS was swapped cause the car just wasn't responsive to gas pedal information after a long drive. The dark thing tried to accelerate wide open while I was waiting at a light and would lag greatly, like pedal to the floor before the car would jump up and go. Since replacing that, the car has definitely become more responsive.

The gauge reads very little difference in gas pressure during WOT. Highest was around 5 PSI lower, but didn't want to keep checking due to being near KC on a Saturday. Cops love sports cars on Saturdays around here. lol

Now, some history. The car sat outside for 3 years before I got it. Tires were from 1992 on it. 2 spark plugs had completely snapped leaving the wires dangling in the engine compartment. The belt was cracking and the alternator was dead. Plus, poor thing was full of leaves. MO weather is abusive to poor cars left in the elements. That being said, I don't know when the last time the coolant was changed. I don't know when the last time the O2 was swapped or the temperature sensors. I do know of the stuff listed and a few vacuum lines that crumbled in my hands, plus brakes and most of the fluids. I don't know of most of the electronic sensors and valves.

I really do appreciate the help guys. I really do. Any information you can share about these cars or leading me to books/pages/and anything it helpful. I have a "Clintons Manual" that helps a bit, but seems to not really address the L98 engine versions of this car with any depth. Lots of info on the 90s ones though and you are nearly left dry for a 84 in it.

Wooddavid88

Oh, and Cats were replaced because the car would bog down greatly and become really unresponsive to gas pedal at around 1400RPM. Took them in for a check at 3 different shops and all of them tested and said the same thing. They were packed. when the guy cut them off and let them fall on concrete they didn't even make a metal sound just a heavy clunk.

To my understanding this car has only one O2 before the cats to monitor for fuel trim, there isn't one behind the cats to monitor cat performance. I don't know if constant back pressure could have harmed it since the check engine light has only now started popping on with long drives around 55 to 65 mph and for over a 20 min drive and is only showing an O2 lean condition error.

Sorry for all the long windedness. I am hoping to give you guys enough information to give you a clear picture of what I am experiencing.

aklim

I'm not 100% sure but I think that is the right size.

Mine does too because it is opening up and sucking air in. It is a "metered air leak", if you will.

Hard to say. Never used propane myself. IDK if it works for larger leaks as well as smaller leaks.

If the injector is leaking, why do you even imagine it will hold pressure that long?

I'd buy an infrared thermometer. They are handy to have and not expensive at all. Harbor freight sells them from $12 to $30.

Yes, you do have a MAF.

Not sure. I have never used WinALDL do I can't be sure what it does.

When I crack open my throttle, the IAC count also goes up and it does make a sucking sound so I think that is right. What I need to know is what your IAC count is when the engine is at stable operating temp with everything but the motor off. It should be about 30 counts. That gives it some room for adjustment. If situation demands, it can up the count rate to compensate for AC or cold conditions, etc. If not, it can lower the count which is why you want about 30 counts or so. If it is higher, it means the ECM is commanding more "air leak". This means you turn the adjustment screw in, rev and recheck. If it is settling at 0, it means there is too much air. Turn the adjustment screw out, rev and recheck. If the screw is out all the way and still 0, you have a leak somewhere. This is assuming your timing is spot on.

How is the sensor? Is it moving properly or constantly stuck at one reading?

Sounds like the pump is delivering.

Dump coolant, take the knock sensor out and the other rubber line on the opposite side. Flush the block and fill. Change the O2 since it is probably lazy after being that old.

Chilton's is very handy for wiping your *** after you take a dump but not much else. I'd get an OBD1 scanner, infrared thermometer, fuel pressure gauge, compression tester, fuel pressure tester if I was buying such a car for the first time. These tools will come in real handy in the future. Also a factory shop manual for that year and not Chilton or Haynes. The paper is too rough for *** wiping, at least for my candy *** but others might be fine with it.

aklim

89 has one O2. You can check if it is lazy or just replace it

Wooddavid88

I am thinking about doing that anyway. I did notice a small coolant leak today near the throttle body. Small section of hosing. Noticed the coolant had hit what looks like a sensor on the very front of the engine near the water pump right underneath the TB. Will probably swap that hosing while I have everything apart for a flush there.

The manual did have some interesting information if you were working on an LT series, but leaves out a lot of info about L98 and Crossfire.

I am thinking of just replacing the EGR too while I am at it. Those torques bolts though... Not growing anymore fond of them as this project goes on.

There seems to be a lot that can cause a rough idle with these.

The reading for the IAC, which I will recheck, is always way above 30 most of the time while idling. Normally something to the tune of 80+ and maxes out at 160. It tends to like to hang, warm or cold, around 120 to 130. I am going to assume this means that either the butterfly valve is too close to being shut, since it is clean, so I may have to set the idle screw and see where that leads or the IAC is starting to fail and could also be helping whatever is causing the lean condition.

Unfortunately the 12 pin ALDL adapter to USB, though a lot of information, doesn't really help me if I don't know what all the readings are suppose to mean and not a lot of people, from what I have found, have posted the numbers for the 89 C4. So... kinda like looking at an Italian word map and trying to find your way home.

Thank you for the help. I will get back on here after changing out the sensors and see where it leads. Could take me a bit, but I will report back either way if it works or not. Just want this girl to be happy and healthy.

Wooddavid88

And, I may now have a new source of toilet paper. lol Maybe some good windshield wipes.

aklim

That could be your coolant from the IAC housing to heat up the TB. You can do the coolant bypass by inserting a pipe instead of having it go through the TB. Still, I'd be curious if your hoses are leaking.

Turn the screw in, rev and see where it settles down. If the EGR were open you'd have too much air. This means that the ECM does not think there is enough air. On a hot engine with everything but the motor off, it should settle between 20 and 30.

Check timing too.

Wooddavid88

By the way, any guesses as to the mystery smoke. It looks like regular exhaust, no color, if you hit it with a flash light you can't see it. So, not white, blue, black, green, purple, or smurf... Just looks to be exhaust. Only appears when I push the car while it is under 160F and only at around 2k RPM. Quick rev or slow doesn't seem to change duration of smoke or valume of smoke appearing. always on the passenger side near the firewall behind the engine block.

My only two guesses are valve cover gasket (the car is nearly 30) or something with the EGR system, but would be hard pressed to know why a system that is disabled while cold would be actively pulling exhaust and only leaking with those parameters.

Also, ambient temp doesn't change the formula. Does it hot outside or cold. Only ever does it once a day too, first start up.

One club member said it was normal for these to do it due to the "cold start injector" turning the system rich to heat up the cats, but I wouldn't know. No one seems to mention it. I am wondering if it might be related to the rough idle too.

I will have a laundry list of things to get done these coming weeks on this Vette. I do love her so the labor of love continues.

Wooddavid88

If you don't mind my asking, what would be the benefits of a coolant bypass? Would it be to get a more dense air mixture to provide a slight boost in performance for the TPI system because of the colder air not being heated by the Throttle Body? (sorry I keep putting "Throttle Body instead of TB. I am a bass player and a website called Talk Bass is also referred to as TB so it throws me off why the Corvette community would keep referring to heating, cleaning, and modifying the bass community until my mind wakes up a bit... mmm coffee.)

JrRifleCoach

Might not be part of your problem, get rid of the K&N air filter!
The L98 fine wire MAF does not like oiled filters.

Remember the O2 sensor has "control" of fuel/air during closed loop.

Good Luck

aklim

Last I heard 88 was the last year of the cold start injector. AFAIK, it was there for starting not to heat up the cats.

aklim

It heats up the TB to overcome possible icing up of the TB in extremely cold weather. Weather which most of us won't encounter. It might help in the way you thought but I doubt it would make much of a difference. The nice thing about bypassing it is that when you take the TB off, you don't get coolant spilling and you can clean the TB easier.