87 vette wont idle

[clok1966]

Im running out of ideas. My 87 runs fine, but it simply wont idle. I replaced the IAC once and it acted just like this. runs and drives fine but when coming to a stop it wont idle, it will drop to say 200 which is just not enough to stay running, the computer will try catch up but its to late. If you start it and very easily keep the throttle at about 500 and let off it will idle. its when letting off at speed it wont stop at 500, it drops to low. As its a manual its a nasty handful in town. As it was running fine and just started doing this i can rule out out a few of the common issues .. lose throttle shaft, replaced the IAC 2X now, same with both new ones. Checked for broken or cracked lines (vacuum) and can find any. Also the EGR is working fine.
I have tried a few Vette forums but (no offence) those guys seem to just buy um, not work on um. I do all my own work and this one got me stumped.. so i figured i would throw it up here and see if anybody had an idea. All in all its pretty much exactly like the IAC is not working.. and it could be, but its not the actual unit, maybe whatever controls it, but not it.

 

[NutBucket]

One trick I've heard to find vac leaks is to spray starter fluid at the lines and such and see if there's any effect. If there's a leak the engine should rev up. Visual inspection for leaks is so tough.

 

[Jimzz]

Need to find someone with a ODB1 scanner. See what each sensor is reading and be able to narrow your search.

Have you cleaned the mass air flow sensor off? Checked the runners and plenum gaskets for leaks? Any engine codes?

 

[RLGL]

What is the Ign system? Delco EST? Are there any codes? When this happens where is the timing? Base engine in good health? Leak down results? The IAC is an output from the computer, an input or lack of is the root cause.

 

[boomerang]

I'm thinking throttle position sensor. The computer still thinks the throttle is open too far for the IAC to take over. Once the computer senses the RPM's have fallen too low and tries to control the RPM's with the IAC, it's too late. Hooking up a scanner as was said would save you throwing parts at it to fix the problem.

Read this.

http://www.obd-codes.com/faq/read-gm-2-digit-obd-codes-free.php

Putting the computer into diagnostic mode will cycle the IAC. With it removed from the throttle body you should be able to see the pintle cycling in and out. With the engine running, and terminals A and B shorted, you can check the base timing if you wish.

 

[deadken]

I think Boomerang is correct. I typed this reply last night, but the servers shut down for maintenance and I couldn't send it:

I'd suggest you check the TPS. I had one go bad on my Bonneville and I would have swore it was a throttle body / IAC problem. The TPS is just a wound resistor and if there is a problem down near the idle portion, the resto of the range can work fine. IOW: the engine can drive fine off idle, but the ECU doesn't have the time to correct when the throttle shuts and the TPS didn't show it closing.

 

[Sluggo]

I'm thinking throttle position sensor.

That would be my guess as well.

 

[clok1966]

scanned, no codes, yes have had it all apart, throttle body etc. (not runners) and cleaned it all up. Ign is standard, not that horrible water pump drove system (thank god). RLGL might be on it, i had read a bit it could be that, but after IAC purchases i was hesitant to do to much , "buy and replace" till something happened fixing the guys who added to that, agreed.. If weather warms up a bit i will update shortly (later this week) will try that.. going to feel dumb if its that.. i was parked last 3 months of summer and its these simple .. update soon.

 

[Vetterin]

If this is something that JUST happened check for bad gas. If you have been messing with your engine re-check your TPS voltage.

 

[boomerang]

Hey did a little research online and the consensus seems to be the MAF. The way to diagnose it is very simple. Remove the connector at the MAF sensor and start the car. If it idles, you have your culprit. If that's the case you may want to try cleaning it before replacing it. There are spray cleaners for cleaning the very fine wire or wires in it but I have used spray electronic cleaner in the past. If it's all gunked up, you might want to take a look at the way your air cleaner is sealing or for a tear in the snorkel running from the air cleaner to the MAF.

http://forums.corvetteforum.com/c4-tech-performance/3249306-1987-wont-idle.html

I Googled "1987 Corvette won't idle" and found two different hits talking about the MAF before posting this. There are probably more. Please post back with your results clok. I'm always interested in feedback.

 

[clok1966]

Bought a new house last year (without a garage) so its outside with car cover, this was last summer and its starting to warm up so been thinking of fixing. Was going to work on it last night as it was in the 40's but the car cover is froze to ground I will post here for sure when I try stuff, nothing like a thread with "fixes" and the OP never posts if they worked or not and the thread just dies. The MAF, im pretty sure I did unplug it to check it last summer (but not 100% so will try that again too).. Im leaning to the TPS as I haven not messed with it, and almost all the rest I have done. Id you where googleing, you might even have hit a thread or two of mine on those vette forums Unfortunately the 40's are now at least a week away again and i need some melt time to get that car cover off without destroying it.

and yes it just happened ( last summer) had drove the car a few times early in spring, did a total clean, popped the top, filled the tank on a 90 degree day and went for a cruize, got across town and started. Just like the IAC (replaced that 2 years earlier) exactly, SO i replaced it again. For a minute or tow I thought I had it fixed, but a short drive and no.. (im very scared something is killing my IAC's and I may need a new one again, as I haven't checked voltages yet). New house, and such the car got put on back burner last year till late fall when i dug in and cleaned stuff up and started checking, snowed, and it got left till this year. SO gas was not bad when it happened, but is almost a year old now. I did start it Nov (as i say it runs and starts fine if I control the throttle) and ran it bit.

 

[Vetterin]

filled the tank on a 90 degree day and went for a cruize, got across town and started. I did start it Nov (as i say it runs and starts fine if I control the throttle) and ran it bit.

That is exactly the reason I think you have bad (probably water) gas. Either siphon out the gas or add a bottle of heet and some stabil to the gas and take a good, highway drive and then see if the idle returns. FWIW, I also have a vette (85) and have been a member of corvettefourm.com since 92.

 

[boomerang]

That is exactly the reason I think you have bad (probably water) gas. Either siphon out the gas or add a bottle of heet and some stabil to the gas and take a good, highway drive and then see if the idle returns. FWIW, I also have a vette (85) and have been a member of corvettefourm.com since 92.

How does bad gas explain problems only with the idle speed? He states it runs normally otherwise.

 

[Vetterin]

Because water in gas WILL run under load but will stall out at idle. I ran a bad tank full from Green Bay to Chicago in January about 15 years ago and it ran great at 70 mph but stalled out at every toll booth and stop. When I got home the tank was near empty so I filled up and all was fine. Sometimes it will screw up your gas filter also.

 

[bruceb]

Check your fuel pressure and also the fuel pressure regulator (usually at the end of the fuel rail)

 

[tortillasoup]

Hey did a little research online and the consensus seems to be the MAF. The way to diagnose it is very simple. Remove the connector at the MAF sensor and start the car. If it idles, you have your culprit. If that's the case you may want to try cleaning it before replacing it. There are spray cleaners for cleaning the very fine wire or wires in it but I have used spray electronic cleaner in the past. If it's all gunked up, you might want to take a look at the way your air cleaner is sealing or for a tear in the snorkel running from the air cleaner to the MAF.

http://forums.corvetteforum.com/c4-tech-performance/3249306-1987-wont-idle.html

I Googled "1987 Corvette won't idle" and found two different hits talking about the MAF before posting this. There are probably more. Please post back with your results clok. I'm always interested in feedback.

I've very carefully cleaned MAFs with a piece of cotton attached to a copper wire dipped in rubbing alcohol. I find my solution to be the most effective way to clean a MAF.

 

[slag]

Because water in gas WILL run under load but will stall out at idle. I ran a bad tank full from Green Bay to Chicago in January about 15 years ago and it ran great at 70 mph but stalled out at every toll booth and stop. When I got home the tank was near empty so I filled up and all was fine. Sometimes it will screw up your gas filter also.

yeah.. No. It would stutter and hesitate if the gas was that bad under load as well, not just at idle.

Your problem wasn't bad gas.

 

[SpatiallyAware]

Everything I've heard and read has been that if you touch that sensor with cotton it snags on the little probe and easily destroys it.

I've only used maf cleaner, and have had pretty good results. The one on my ranger needs cleaned every couple years, I've been spraying it with the maf cleaner for going on 10 years now without a problem.

 

[Squeetard]

MAF or Throttle position sensor are the most likely culprits. AFTER WIRING. Check all the connections.

 

[boomerang]

yeah.. No. It would stutter and hesitate if the gas was that bad under load as well, not just at idle.

Your problem wasn't bad gas.

Agreed.

 

[boomerang]

Everything I've heard and read has been that if you touch that sensor with cotton it snags on the little probe and easily destroys it.

I've only used maf cleaner, and have had pretty good results. The one on my ranger needs cleaned every couple years, I've been spraying it with the maf cleaner for going on 10 years now without a problem.

My former DIL had a car, the name of which escapes me now, that had clean air for PCV purposes drawn from the snorkel at a location that was aft of the air cleaner but before the MAF. Consequently, over time, fluctuations in crankcase pressure would make a mess of the interior of the MAF. She had a horrible hesitation in the car and when I physically looked at the MAF, there was what I would term "wool" clung to the wires.

 

[clok1966]

Well took longer to warm up then expected (yeppie North Dakota, still like it here, but man spring is long time in coming this year). Replaced the TPS and a very quick test it seems to have fixed it. My backyard is a bit "damp" no way I can get it out yet and actually drive it. I must say sitting all winter, this thing starts better then any car i have ever had. Not even a crank, it fires the split second it cranks. Put the NEW tps on and ran the same, some adjustment and it was running fine and not dying at all. Wont be 100% sure till i can drive it (and will update this once again) but it seems it is solved.

Old gas, yes sadly it had a full tank when this happened (heck i made $40, gas has went dang near 2 bucks since i filled it) But I do use stable, and its always had fuel line antifreeze in the gas (nd, you use it on everything). Surprisingly it doesn't smell old and seems to be fine. I will say Im 99% sure it not water in gas, but never say never. If it still has issues when i test drive (week or two yet if we can stay warmer and dry) I will look at that. My motorcycle on the other had with year old gas smells so bad running i can hardly stand it)

 

[Vetterin]

Well, I'm glad you're making progress. Hope your problem is solved. I sure got to agree with you on the starting capabilities of these early C4's. I installed a quick disconnect on my battery a few years back for long storage. I didn't even garage my vette this winter and when I went to start it this spring it probably took 3 seconds to turn over.