I'm having weird idle issues with my 87 Vette. At cold start up it'll idle at 1000 rpm then drop to 700 after about 10 to 20 seconds which is normal. It's not a steady idle though, it's sporadic but stays inbetween 600 and 799 rpm.

I recently replaced my intake gaskets and fixed a MAJOR idle issue I was having and it ran excellent for about a week and a half and now this is unsteady idle is happening again. The only thing I've changed since then would have to be spark plug wires, went from ACDelco 7mm (from 8mm Accel) that were from my Camaro I had a few years back, they had not even 10k miles on them. Plugged up a vacuum port that came off of the air pump assembly, not sure which vac line but it's no longer a leak, and plugged a vac line that went to the three way check valve. I think I blocked the line that goes to the accumulator for vac. Obviously the wires could cause a sporadic idle but they're good wires as best to my knowledge.

My spark plugs all have different gaps, no idea how it happened due to the fact I put them in myself, slight change tey we're mishandled because a friend was helping me. A few could have been dropped without me knowing (ACDelco 41-800 for aluminum heads) they were replaced about four months ago and it didn't have an idle like this when my plenum rebuild was done.

It feels like it's a vacuum leak of some sort. As long as the car is rolling the RPM stays constant but once I come to an idle at a stop my seat wants to vibrate off of the rails.

I appreciate any ideas, I'm stumped on this one. I have half a mind to redo all I replaced and see if that fixes anything.

 

So not only is the Idle speed hunting the engine misses so bad it shakes ur drs seat?
at a stop my seat wants to vibrate off of the rails.

Man you got all kinds of things going on here.

First plugs were replaced and could of been dropped and all with different gaps? and you have installed Used Wires?

Both of those issues will cause a bad miss and should be addressed first.

Got to start with some fresh plugs gapped right to rule out any broken/cracked porcelain's and they need to be put in there without dropping any and without cracking any while tightening.

Those wires should be sprayed from a water bottle to see if it affects the engines idle at all while the motors running at Idle.

Then taken off one by one and checked for resistance while wiggling the crap out of them everywhere and from the ends.

When you had the Intake off did you remove the throttle body for a good soak? and also remove the IAC and clean out all of its passages? then reinstall with new gaskts? If you have carbon ledge in the TB down where the butterfly rests at Idle. It blocks the small amount of air that should past there for a smooth Idle and cause the IAC to hunt.

How is it when Hot? Same? I mean does it hunt, Hot?

Have you re-torqued the Intake Hot? Have you sprayed carb cleaner everywhere there could possibly be another vacuum leak?

Is this thing stock/modified in any way.

You replaced the Intake Gaskts and fixed a major Idle Issue? and it's happening again? Is it doing the same exact thing minus the miss?

Have you ever cut the heads? were they milled? That opens up the intake valley and can cause Intake sealing Issues if you don't use the Blue on the Intake gaskts

 

forgot to ask him his political affiliation really great advice

 

Thanx's

 

I apologize for the jumble lol I was typing that on my phone with only 3% battery life to spare, had the get the message out.

The wires are just a temporary fix, unfortunately they were all I had due to three of my 8mm wires getting burned. They came out of a 1991 Z28 305 Camaro. The plugs have about 5k miles on them now and are just starting to show some white lol. I'll remove and regap them.

As for the intake "rebuild" yes, everything I could get my hands on was cleaned. I even used a parts washer and about 5 cans of carb clean. The throttle body was thoroughly cleaned except for the IAC which was a dumb mistake but I was on a time crunch. I guess removing the bottom portion of the throttle body isn't too difficult to do, I can clean it out this Friday. Electric parts cleaner and carb cleaner?

When it's hot it is not much better. Seems to calm down a bit but I can still feel the slight hunting.

I have not tried re-torquing the intake manifold bolts. I used my service manual and followed specifications on torque pattern and pounds. Then after 24 hours I retorqued. Today I did retorque the runner and plenum bolts and man were they loose. About two full turns out. Something tells me my manifold bolts could be loose, something i'll check tomorrow.

This thing is completely stock, all except the K&N air filter.

Again, sorry for the choppiness of the post lol. To clear things up I was having a bad vacuum leak due to the intake manifold gaskets being shot. Once the new gasket was installed the car idled and ran/shifted like a dream! Idle was solid for about a week and a half. Since last week it's been steadily getting worse with the idle issue.

That really is a good point about tightening down those intake bolts hot. I didn't think of that. I'll have to recheck my torque on those bolts if I can get to them without having to take the plenum apart. They could have worked themselves out and starting to recreate my vacuum leak issue like before. Though I did put blue loctite on the bolts

Hopefully it's something as simple as me being negligent

 

Sounds like the Intake to me. Spray around it and see. Crows feet and skinny 1/4 sockets with long extensions with flex's and just snug-em up, then just a little more. When hot.

You used non hardening sealer on the intake bolts, right. Everything except the corner ones don't have a bottom and it helps the bolts hold.

I wouldn't mess with anything else right a way for now

Good Luck. Lets us know how it turns out

 

I'll tackle that in the afternoon for my lunch break lol. I should have known that might be the culprit. When I put my foot to the floor I smell burning oil about 5 seconds later. Plus my fresh coolant had new oil in it. I just assumed it was old stuff caught in the line.

I really do appreciate the help! I'll let you know if that's the fix

 

Check timing first to rule it out. Also get all the plugs and wires straight. Make sure cap and rotor is good and doesn't have boogers in it. Check for air leaks. Unplug each hose and stop it off to see if it changes anything. Finally, take the air hose off the throttle body and stop off the IAC and see what happens. If the engine doesn't die, you have an air leak somewhere.

 

I was going to torque those intake bolts back down until I realized I can't get to half of theme because of the runners. Is there any way around them or am I screwed with having to take the top plenum off again..? My bolts are the original star bit. After further inspection I'm noticing oil around the head of a bolt. Not many ways it could have gotten there lol

 

You can do it once right or do it again. Me, I'd do it right the first time

 

I did everything by the book. It stated nothing to retorque once hot. But I should have thought of that due to thermal events causing things to expand lol.

Tomorrow I will be removing the upper plenum and re torquing those intake bolts. I'm assuming they finally shifted enough to leave a small gap for a vacuum leak, no biggie.

 

I'd try to find out where the leak is before I go into that. Once you know and do it, maybe you can see whether the leak fixes it?

 

Man go easy on the Guy, hes trying. Live and learn. Whom here hasn't tried and failed, even when you did everything right

I'm assuming he did what I said and found it leaking air by spraying around to confirm and anyways he needs to reseal the bolt threads with non hardening crap

Don't worry dude. It happens to the best of us. Most of the time, its the ends of the manifold that get-cha and when that happens, it's a mess and a redo.

Remember to go in Sequence, I think the real early ones calls for the center out and then when the aluminum heads came out, they call for side to side. Remember it's like 25 pounds max.

Get it done and move on to your Miss

 

I'm not giving him a hard time. I'd want to know exactly where it leaks and not just a general area so that in future, if there is a leak, he can find it quicker and/or know how to make sure it doesn't occur again. Just torquing down the manifold without investigation is not a good idea. I really hate the quick and dirty fixes without knowing why it happened. Fixing the problem without knowing why isn't enough. Why it happened is more interesting. Could the manifold not be sealing properly because it isn't flat? IDK. Could there be more than one leak?

 

My book said all of the left side, then the right side for torque sequence. 25 to 45 pounds.

I DID NOT retorque the bolts after I ran it hot. The plenum was doing a lot of expanding and contracting causing it to not seat right and now the bolts need to be tightened down again. The exact thing happened with my runner bolts. I know for a fact that I torqued my runner bolts down when the rebuild was done. When I checked them a few days ago they were loose again. I am getting oil coming from the front bolt and oil in my coolant again. Worse comes to worse I'll replace the bolts with the same tensil strength but with 13mm heads instead of torx bit.

After this second tear down I'll be pretty fast at taking those runners off haha! This whole idle issue makes sense now that you guys through your two cents in. It feels the same as it did before the vacuum leak, now that it ran excellent for two weeks and progressively got worse, the plenum was expanding and contracting. Now that it's at it's highest seat point I can retorque the bolts down. Should only be a three hour job.

 

So Plausible.

 

I'm not going to be able to remove the intake today. Family emergency.

I did however respray to try and find a vacuum leak. What's the boiling point on carb clean cause when I sprayed it on the intake between the intake gasket and the head it starts bubbling immediately. I got a slight idle change on the third bolt on the driver side. Then again that's all it really takes to have a leak and rough idle.

I did ask my instructor about my issue this morning and he said that the intake wouldn't move that much to cause an issue like this. Then again it does make sense that thermal expansion occurs causing bolts and the intake to expand. I'm noticing more and more it idles better when it's hotter which means the intake expanded enough to block up the leak a bit, right? Plus the oil in my coolant again and he oil leaking onto the manifold, there's no other way for that oil to get to where it is unless it's from the bolts or gasket itself. Its been one frustrating day I'm just trying to get this figured out, let my mind at ease. Obviously I won't be able to do work on it until next week, maybe Tuesday.

 

I'd like to redo the gasket.