Need help with poor idle
05-11-2010, 01:11 AM
#1
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Need help with poor idle
As the topic says... my idle sucks.
Car has been on stands for about 7 months. I have adjusted my valves, and changed out some exhaust components - I now have a cat and primary bypass, stock secondary. I replaced the spark plugs, dizzy caps, rotors, and coils.
After starting the car the idle starts at approximately the right point - perhaps a bit low, say ~800rpm. From there it will slowly and consistently drop until the car dies. It makes the (very occasional) pop as I let off the throttle - has happened maybe 3 times in ~10 minutes of run time.
I have checked the basic spots for vacuum leaks - reservoir is there and is holding vacuum, all the other connections seem good. Reseated the intake manifolds and am fairly confident they are fine. Looked over the exhaust also, appears to be leak free.
Revving the car everything seems good. Disconnected the oxygen sensor, and the car will not idle at all, so it seems to be working. ISV recently cleaned.
Running out of ideas here. I'm assuming my gas is well past its prime, but I'm not confident that would cause these issues...?
Car has been on stands for about 7 months. I have adjusted my valves, and changed out some exhaust components - I now have a cat and primary bypass, stock secondary. I replaced the spark plugs, dizzy caps, rotors, and coils.
After starting the car the idle starts at approximately the right point - perhaps a bit low, say ~800rpm. From there it will slowly and consistently drop until the car dies. It makes the (very occasional) pop as I let off the throttle - has happened maybe 3 times in ~10 minutes of run time.
I have checked the basic spots for vacuum leaks - reservoir is there and is holding vacuum, all the other connections seem good. Reseated the intake manifolds and am fairly confident they are fine. Looked over the exhaust also, appears to be leak free.
Revving the car everything seems good. Disconnected the oxygen sensor, and the car will not idle at all, so it seems to be working. ISV recently cleaned.
Running out of ideas here. I'm assuming my gas is well past its prime, but I'm not confident that would cause these issues...?
05-11-2010, 04:03 AM
#3
Nordschleife Master
In these situations where it was find before, I always go back and check what I've changed. From your list I would pull the dizzy caps and check that the rotors are seated correctly. It could be that one is not fully pressed home and only making partial contact with the poles in the cap (been there and done that myself).
I would even consider putting the old coils back in - it's not unknown for a new coil to be bad.
I would even consider putting the old coils back in - it's not unknown for a new coil to be bad.
05-11-2010, 08:45 AM
#4
Burning Brakes
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Dear Xeps
If I understand you correctly, initially idle is OK other than it being a bit low - am I correct? If that's the case then the only things that really changes as the engine runs and the idle degrades is the engine temperature increases - maybe something to do with head temp sensors? Fuel is flowing and possibly debris in the fuel filter is restricting flow - does the engine run OK when above idle? Since you've replaced almost everything ignition related other than the spark plug wires, I have to ask how old are they and why they wern't replaced?
Bill
If I understand you correctly, initially idle is OK other than it being a bit low - am I correct? If that's the case then the only things that really changes as the engine runs and the idle degrades is the engine temperature increases - maybe something to do with head temp sensors? Fuel is flowing and possibly debris in the fuel filter is restricting flow - does the engine run OK when above idle? Since you've replaced almost everything ignition related other than the spark plug wires, I have to ask how old are they and why they wern't replaced?
Bill
05-11-2010, 11:10 AM
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Dear Xeps
If I understand you correctly, initially idle is OK other than it being a bit low - am I correct? If that's the case then the only things that really changes as the engine runs and the idle degrades is the engine temperature increases - maybe something to do with head temp sensors? Fuel is flowing and possibly debris in the fuel filter is restricting flow - does the engine run OK when above idle? Since you've replaced almost everything ignition related other than the spark plug wires, I have to ask how old are they and why they wern't replaced?
Bill
If I understand you correctly, initially idle is OK other than it being a bit low - am I correct? If that's the case then the only things that really changes as the engine runs and the idle degrades is the engine temperature increases - maybe something to do with head temp sensors? Fuel is flowing and possibly debris in the fuel filter is restricting flow - does the engine run OK when above idle? Since you've replaced almost everything ignition related other than the spark plug wires, I have to ask how old are they and why they wern't replaced?
Bill
Basically what happens is this: turn the key, car starts immediately, no excess cranking. After the initial surge from starting, the idle essentially "starts" at ~800rpm, and just slowly decreases until the car can no longer run. This process takes maybe 10 seconds. The idle does not hunt at all - it starts at ~800, and the needle only slowly moves downward - never upward unless I hit the throttle.
If I give it some gas, the car runs well with one exception, that it will occasionally make a slight pop when coming off throttle. No erratic valvetrain noise, no pinging, etc. Idle microswitch appears to engage fine. ISV is clean, as is the AFM.
05-11-2010, 11:13 AM
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In these situations where it was find before, I always go back and check what I've changed. From your list I would pull the dizzy caps and check that the rotors are seated correctly. It could be that one is not fully pressed home and only making partial contact with the poles in the cap (been there and done that myself).
I would even consider putting the old coils back in - it's not unknown for a new coil to be bad.
I would even consider putting the old coils back in - it's not unknown for a new coil to be bad.
05-11-2010, 05:41 PM
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Been tinkering with this today...
Car runs equally poorly on either coil. Does not idle with ISV unplugged. Cleaned ISV, idle seems better - the "Rate of decay" seems to have slowed, but the idle is still very low. In fact, on occasion (every third try or so) it will hold the idle for a few seconds. Needle stops dropping, but then inevitably it finds its way downward again. Needle never moves up, no surging or hunting is occuring.
Also traced my spark plug wires - they are correct. Checked distributor rotors, they are snugly in place. Gave the intake manifolds another once over, and they look fine. Checked the exhaust, all connections seem tight.
Car runs equally poorly on either coil. Does not idle with ISV unplugged. Cleaned ISV, idle seems better - the "Rate of decay" seems to have slowed, but the idle is still very low. In fact, on occasion (every third try or so) it will hold the idle for a few seconds. Needle stops dropping, but then inevitably it finds its way downward again. Needle never moves up, no surging or hunting is occuring.
Also traced my spark plug wires - they are correct. Checked distributor rotors, they are snugly in place. Gave the intake manifolds another once over, and they look fine. Checked the exhaust, all connections seem tight.
Last edited by xeps; 05-11-2010 at 06:19 PM. Reason: update
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05-11-2010, 07:40 PM
#8
Nordschleife Master
I guess it was standing with the battery disconnected for a long time but it still might be worth trying a DME reset:
https://rennlist.com/forums/964-1989...dme-reset.html
Costs nothing and it worked for a friend of mine who had a bad idle.
https://rennlist.com/forums/964-1989...dme-reset.html
Costs nothing and it worked for a friend of mine who had a bad idle.
05-12-2010, 12:23 AM
#10
RL Technical Advisor
A few thoughts,........
1) Never "***-ume" anything. You need to perform some tests to ensure certain components are properly working as well as check for vacuum leaks.
2) Check the oxysensor to ensure its cycling correctly. Measure the lambda voltages and note how quickly they change. If the oxysensor has more than 50K on it, it may be time for replacement.
3) Each and every intake runner, rubber fitting and such must be checked for vacuum leaks as even the smallest one will play havoc with the idle. We use either a smoke machine or careful use of carb cleaner to check everything in the whole system. Attention to detail is required,......
3) Naturally, the electrical and ignition system must be perfect. This means ALL grounds, alternator, voltage regulator, battery cables, etc. Once again, close attention to detail pays dividends.
4) Check the idle switch and FT switch on the throttle body. That means wiring and connectors, too.
5) Clean that ISV with carb cleaner, I just went through this with a customer who had similair complaints. He had "cleaned" the ISV several times but the problem persisted until I hollered at him about how to do this and what to use. After he followed instructions, that nice smooth idle was restored.
This stuff can be frustating but its all fixable with patience and a methodical approach.
1) Never "***-ume" anything. You need to perform some tests to ensure certain components are properly working as well as check for vacuum leaks.
2) Check the oxysensor to ensure its cycling correctly. Measure the lambda voltages and note how quickly they change. If the oxysensor has more than 50K on it, it may be time for replacement.
3) Each and every intake runner, rubber fitting and such must be checked for vacuum leaks as even the smallest one will play havoc with the idle. We use either a smoke machine or careful use of carb cleaner to check everything in the whole system. Attention to detail is required,......
3) Naturally, the electrical and ignition system must be perfect. This means ALL grounds, alternator, voltage regulator, battery cables, etc. Once again, close attention to detail pays dividends.
4) Check the idle switch and FT switch on the throttle body. That means wiring and connectors, too.
5) Clean that ISV with carb cleaner, I just went through this with a customer who had similair complaints. He had "cleaned" the ISV several times but the problem persisted until I hollered at him about how to do this and what to use.
After he followed instructions, that nice smooth idle was restored.This stuff can be frustating but its all fixable with patience and a methodical approach.
05-12-2010, 09:53 AM
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Happily, I have solved this issue.
As I mentioned in one of my previous posts, I had been going over all the manifold connections, making sure the hose clamps were properly centered, properly tightened, that the manifold was well seated, etc...
Turns out the problem WAS with the manifold all along. I recently had them powdercoated, and for prep I had removed all the extraneous bits from the outside of the manifold, including a little rubber nub from the back of the drivers side manifold.
What I failed to realize is that piece goes completely through the manifold - meaning I had a ~3/8" hole hiding right at the back of the manifold. Surprisingly, I couldn't hear the air movement. Covered it up with a finger and idle returned to normal. Have replaced the small bit and all is good now!
Now to take care of this...
As I mentioned in one of my previous posts, I had been going over all the manifold connections, making sure the hose clamps were properly centered, properly tightened, that the manifold was well seated, etc...
Turns out the problem WAS with the manifold all along. I recently had them powdercoated, and for prep I had removed all the extraneous bits from the outside of the manifold, including a little rubber nub from the back of the drivers side manifold.
What I failed to realize is that piece goes completely through the manifold - meaning I had a ~3/8" hole hiding right at the back of the manifold. Surprisingly, I couldn't hear the air movement. Covered it up with a finger and idle returned to normal. Have replaced the small bit and all is good now!
Now to take care of this...
05-12-2010, 04:00 PM
#14
Rennlist Member
