Idle problems

I've got a 1985 that varies its idle RPM from 550 to 750, in park and in drive. Car runs great above 750 rpm. New plugs, cap, rotor, and MAF sensor. Can't detect any vacuum leaks. I was told that the 1985 did not have a power or burn-off relay for the MAF sensor. Any ideas. Thanks. 

Vacuum leak. 100% probability.
 
http://www.corvetteforums.com/m_37283/tm.htm
 
print it out, put it in the bathroom.

 
My C4 '87 runs at 500 in park or drive(foot on the brake).  Not sure why your's varies...
 
Ask Mech259  he might know...

quote:

ORIGINAL: lbelew

I've got a 1985 that varies its idle RPM from 550 to 750, in park and in drive. Car runs great above 750 rpm. New plugs, cap, rotor, and MAF sensor. Can't detect any vacuum leaks. I was told that the 1985 did not have a power or burn-off relay for the MAF sensor. Any ideas. Thanks. 

The answer to your burn-off relay question is also in the article. Fifth post (p. 23). It won't matter for yours however; we already know  whatcha' got there...

Throttle plates probably need cleaning. When they get carboned up, it is difficult for the plates to seal in throttle bore and the ECM has trouble controlling idle.

Thanks for all of the input. I guess I'll try cleaning the throttle plates next. I read the article and tried the propane with no luck of locating a vacuum leak. The car has 30300 miles on it and is like new except for my idle problem. I've set my idle at 700 rpm in park and the car purrs, but at 500 it surges to 700. What do you guys think of an EGR issue. I currently do not have any codes stored or a service light. By the way I also have a 2000 and a 1979 vette. Thanks. 

One more thing to note, this is a 1985 car that has had about 2000 miles put on it in the last ten years. Thanks.

30K doesn't seem like enough distance for a vacuum leak to develop. I can't imagine carbon build-up either.
 
...or anything else failing, mechanical-wise. Rubber and other organic fiber compounds - vacuum hoses and gaskets, COULD fail, given the time period.
 
So I gotta' stick with an intake leak somewhere. Get back on this one tho'... I'd really like to know on this one.

I agree with cleaning the throttle body and the IAC also.  You can usually hear vac leaks....like a hissing sound. 
 
Does it make a difference if the engine is cold or hot?  Usually a vac leak will disappear once the ECM goes closed loop.  A easy test to do if you have an ohm meter is to check the resistance of the injectors hot and cold. 

the burn off relay is behind the breadbox   check the fuel filter and like said above  clean the plates

quote:

ORIGINAL: RRT vette

  You can usually hear vac leaks....like a hissing sound. 

 
That's a negatory there, rrt.
 
My '87 had a ton of leaks, and the only one I ever heard was the one from the incorrectly seated cold start injector, AFTER top end re-assembly.

Thanks for all of the good info. Cleaned the throttle body and still have the idle problem. No indication of vacuum leaks using the propane method. The erratic idle only occurs after initial warmup and once again if I set my idle at 700rpm the car runs great. I think I'll check the power and burnoff relays next.

quote:

ORIGINAL: lbelew

I currently do not have any codes stored or a service light.

I missed that you got no codes. Confirms vacuum leaks. Nothing else will cause your symptom. Nothing.

Any motor gettin' air that's not mixing with the gas (or not getting 'registered' to mix with the gas), will rev when the air mix leans out.

THEN...

The carb'ed motor pulls more gas (no throttle change), and it revs down.

The TPI revs as the leaner mix passes (no throttle change), the computer gives gas for the revs, and it slows back down - cause no acceleration.

Tunin your carb, you increase the fuel from the jets, the revs go down.

My neighbor's lawn mower did it.

Ya gotta' get a torque-on. The bolts, that is... Either that, or pull the plenum TB, runners, and intake. If you don't, you will get carbon fouling.
 
Gas is not what burns. It's the oxygen. Rocket fuel is liquid oxygen.

Once again thanks for the info. Thought I had it fixed when I pulled and cleaned the Idle air control, but it started acting up again at idle. One thing to note is that my air plenum was pretty sooted up when I pulled it to clean the throttle body. I am picking up a lower and upper intake gasket set today. Any suggestions of what else to check while I have it apart. Thanks.

quote:

ORIGINAL: lbelew

Once again thanks for the info. Thought I had it fixed when I pulled and cleaned the Idle air control, but it started acting up again at idle. One thing to note is that my air plenum was pretty sooted up when I pulled it to clean the throttle body. I am picking up a lower and upper intake gasket set today. Any suggestions of what else to check while I have it apart. Thanks.

Get some parts cleaner. The gallon size, with the basket in it. Get a couple of paint brushes, for paintin' on the parts cleaner. Get a low-rise plastic tub that the intake will fit in. That catches the cleaner fluid as you brush it on. Get a bottle brush for the plenum and the runners - they got a ton of carbon.

Get some Trans-X for the injectorO-rings. If they're not visibly cracked, they'll re-seat just fine if they've been allowed to sit in the Trans-X for 6 hours. AND KEEP A FINGER ON THE INJECTOR CONNECTOR CLIPS WHEN REMOVING, TO PREVENT BALLISTIC DISAPPEARANCE. Someone else called 'em Jesus clips, but don't use his name unless it's from the heart.

You might need an 8" long x 1/4" drive ratchet extension, with a 1/4" x 3/8" adapter on the end, to get the passenger side, rearmost intake runner-to-intake manifold bolt. And possibly the forward-most passenger side bolt also. THEY ARE TOUGH!

Put all bolts back in the original holes as soon as possible, until ready to clean the bolt and hole itself. Penetrating lube with the spray tube works pretty well for the hole. You do NOT  want to mix up bolts. I don't think there are two alike, except the distributor cover (maybe...).

Do a total dry (trial) fit at re-assembly, without gaskets. Trust me.

Put a grease film on all gaskets when you do final re-assembly. And they gotta' be SURGICAL clean. You don't want ANY air leaks. Use metallic thread lube - can't remember the metal. Anybody?

When removing parts, make sure there's NO 'jam' in the bolt heads. YOU HAFTA make sure the socket is well seated in the bolt. Especially in the hard-to reach spots. One 'round-out' in the wrong place is gonna' triple your work time and cost. 

Don't drop any bolts in the distributor hole. Make sure the cold start injector is properly seated. If you miss, it won't take long to find out what's up with that. After you get that one right too, you won't believe the difference.


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Thanks, Ive got a busy night ahead of me.

quote:

ORIGINAL: lbelew

Thanks, Ive got a busy night ahead of me.

Night, huh? You probably could if ya' really wanted. But don't. 

It took me 3 weekends+ ~ 55-60 hours. The dry fit, putting each bolt back in the hole as you go, cleaning out bolt holes with lube, AND A SHOP-VAC (call it over-kill if ya' want), cleaning bolt threads with a toothbrush, and stopping to take pics and write stuff down, really adds alot of time.

But afterwards, you KNOW when you pull the trigger, it's gettin' ready to play SWEET music. 

Unless you didn't properly seat the cold start injector. Only took a minute to figure that out tho'....
 
EDIT:
Put a piece of tape extending from the distributor rotor to the underneath outside of the rotor housing. THEN, before pulling the distributor out, cut the tape between the two. The rotor will turn as you pull it out, independantly of the shaft, and the tape will 'UN-align. It will 'RE-align', as you re-insert. 
 
Also, mark the shaft in relation to the manifold mount hole, and score it with a screwdriver, along with the shaft. You can also put a piece of tape from the base of the rotor housing, to the firewall top. You want to have as many reference marks as possible. And remember how it turns as you pull it out, because you have to turn it back before going back in. the cam drive gears are helical.

Spent a good twenty hours pulling and cleaning the intake system. Still have the same idle problems and now Ive got a little hesitation when putting the pedal down. Still no codes present. One thing to note, when the car is idling erratic and I remove a vauum hose, any vacuum hose, it smooths right out for a couple of minutes before going back to its erratic idle. Any thoughts. Thanks. 

quote:

ORIGINAL: lbelew

Spent a good twenty hours pulling and cleaning the intake system. Still have the same idle problems and now Ive got a little hesitation when putting the pedal down. Still no codes present. One thing to note, when the car is idling erratic and I remove a vauum hose, any vacuum hose, it smooths right out for a couple of minutes before going back to its erratic idle. Any thoughts. Thanks. 

 
If it smooths out for a moment, that seems to mean the computer is readin' the air well (and unfortunately, the leak too).
 
Did you see any places during disassembly where the gaskets were compromised? Worn through? I guessin' you got a different leak now, from somethin' not seated right. ESPECIALLY if no diagnostics codes are tripped.
 
If you can't find it, you're gonna' have to crank the idle speed to get a steady idle speed, and hit it again with a propane torch.

Old gaskets looked to be in good shape with no areas compromised. I'll hit it with the propane again. How do I troubleshoot the computer if the whole thing has been the computer from the start. thanks. 

quote:

ORIGINAL: lbelew

Old gaskets looked to be in good shape with no areas compromised. I'll hit it with the propane again. How do I troubleshoot the computer if the whole thing has been the computer from the start. thanks. 

 
The parts retailer will usually plug in their scanner for free. If you do that, ask them if anything in the computer, and only in the computer, can get by their scanner.
 
Do not ask them about a vacuum leak; they'll say if you can't hear it, there ain't one. And that ain't the case with a TPI. I know ya' don't wanna' hear it after all the intake re-workin', but I think you didn't get two parts seated properly.
 
Was there any part you weren't sure of when re-installin'? At all?
 
The gasoline delivery process, and the ignition (spark) process, by themselves, will not cause the lopin' idle - only airflow process can do this.

I have a code 43 and a code 44. The 44 more than likely is due to the 43. I can read these myself by jumping the appropriate ports in the diagnostics port. First I'll try the propane and if no joy I'll have to replace the ECM BASED ON THE CODE 43. THANKS.

quote:

ORIGINAL: lbelew

I have a code 43 and a code 44. The 44 more than likely is due to the 43. I can read these myself by jumping the appropriate ports in the diagnostics port. First I'll try the propane and if no joy I'll have to replace the ECM BASED ON THE CODE 43. THANKS.

 
I do NOT think they are related.
 
As far as the 43 code, I would NOT spring for a new ECM, until testing for a signal for timing retard. The 43 code says retard has been on too long (which would mean there IS a signal) OR ignition retard system failure (no signal). TEST FOR SIGNAL FIRST!!!
 
If 44 can mean vacuum leak, I'd go that way first. Since both codes COULD be because of ECM connections, and not necessarily a bad ECM, check all the leads with a probe.
 
Even if there's some short circuits to the ECM, you still got the loping idle.
 
Check the service manual for both codes, to see if it says go after one of the two codes first. Or did you already do that?

Just checked it with the propane with no evidence of a vacuum leak. I think I'll pull the ecm and reboot it. Thanks.

Code 43 can be caused by bad knock sensor or broken wire to the knock sensor which is just in front of your starter.