Carrera uneven idle, now what?
 

I am working on a relative's 87 carrera. The car has a slightly lumpy idle. So far, I have done:

1. valve adjustment
2. new spark plugs
3. new dizzy cap
4. new rotor
5. new 3-wire universal O2 sensor

The car will fire up just fine, runs real smooth for a few minutes, and then settle down to a slightly lumpy idle (meaning with some vibration).

No obvious air leaks (I know they can be non-obvious)
I will want to clean the Idle Valve
The airbox has a small crack BEFORE the MAF meter so that should not affect idle

I still have a few other ideas to check but I like some recommendations as well.

The question is what does the ECU do during the first couple of minutes after starting that is different from a few minutes later?

Get that ICV cleaned thoroughly and double check the idle micro-switch.....go from there (at minimum)...first things first.

I wouldn't think that there would be any time concerns, as to the ECU, between starting and a few minutes later,..although I could be wrong. Wouldn't surprise me, for sure.

BEST!

Doyle

I did check the idle switch, it is in correct position so that when it is moved off idle, it slightly clicks.

With some other cars, when the engine is cold, the ECU runs open loop and slightly rich waiting for the O2 sensor to warm up, then it uses the O2 sensor signals to tweak the stoicheometric mixture. But, I do not know about the 87 Carrera firm ware.

Get a Steve Wong chip.

First of all you are introducing possible problems with a universal o2 sensor. Use the OEM one. Then I don't know whether the lumpiness existed before, or happened after you worked on it. You can draw some conclusions here.
If the lumpiness existed before I would do a compression and leakdown before I did anything else to make sure the foundation was good enough to invest time and money into it (not knowing the history of the car). Check spark plug wires with a scope to see if they are shorted or open; Look for about 10kv at idle. If they are all consistantly high (15 or 20) I would check the CO to see if the engine is set too lean. If they are all consistantly low (5kv) I would check the mixture to see if it is too rich.
Use the factory procedure to check CO and a BAR 90 certified CO meter (not gunston)

Put the Olson o2s to see if it cleans it up.

there are couple of thread on this I remember - there is a warmup cycle / loop that I believe is triggered by several things including the O2, the CHT sensor, and one more I can't recall.

I went through the same process you did when I bought mine a couple of years ago. Solution? As a separate mod, I bought a Steve Wong chip which remapped the idle - smoother and raised RPM as well.

there is a method to raise the idle by jumping a test connector while making adjustment, but that is not advised without being hooked up to the gizmo the pro's use to sense rich/lean in the exhaust. Foggy on the details here - worth a search.

I don't know what this means.

A compression test was done by someone else a few weeks ago with very good high numbers and all even across the cylinders. I do not have any of the professional scopes to check high voltage tensions {10Kv} or a BAR 90 CO meter. I do have a gunson and likely not accurate enough for this work.

Sorry, damn auto correct tablet.

Put the old O2 sensor to see if it clears up.

I think it might be the Air temp sensor in the AFM. Ref and Speed sensors figure into the equation as well as the AFM door position wiper.

I got a book on the Motronic engine management system to satisfy my curiosity once, and while I am not expert, I think you hit the nail on the head. While waiting for the O2, the ICV gets adjusted and idle is a bit high, but when things warm up, the ECU adjusts the ICV and the idle drops to normal. I've forgotten a lot of what I read, so hopefully folks will forgive me. Steve's code is much more efficient.

The old O2 sensor was also a spliced in universal sensor and changing it to a new Unversal 3 wire sensor did not eliminate the lumpy idle. Despite its being a universal one, I do not think this is the problem.

I will clean the idle valve next, and check the O2 sensor signal while unplugging the injectors. There is also the head temp sensor to check too.

My 3.2 runs rough when the AFR's are around stoch and leaner. Running around 13.5 AFR seems to make it run smooth as butter.

Maybe the intake is loose (common) or a deteriorated intake gasket or two?

Good points noted here. As to the idle switch, I wouldn't trust the "sound" (click) to sign off on the Idle switch functionality. I'd make use of a VOM to measure it's continuity.

BEST!

Doyle

I had a similar issue and it turned out to be air leaks on the micro thin intake gaskets. I used a propane torch ( unlit!) with a hose to feed gas to the area around each intake. The leaker will pick up RPM. Good luck

Hey,
I recently purchased my '84, El Chup. While the familiarization was taking place, I was rummaging around in the engine bay. I found that all 12 intake runner nuts were loose, as well as the copper/rubber pipe that runs from the back of the post AFM elbow to a vacuum line on the left side of the bay. All these leaks coupled with the stock '84 ECU meant that while in the warm up period it ran fine, but as soon as the warm up period ended it began the lumpy idle phase. My solution was to tighten all the intake runner nuts and to replace the rubber portion of the copper/rubber piping. I then replaced the CHT sensor and installed a Steve Wong chip. All these details coupled together brought the engine to a new level of operation, smooth even idle, and a absolutely linear power production from idle to redline. Previously the the power production was stepped, almost like a carb when the vacuum secondaries opened. Search my posts for a more detailed explanation of some of these fixes.
eric

A lumpy idle will be due to a vacuum leak. Start by pressure testing the intake and looking for leaks or use carb cleaner and spray on vacuum line connections or even just use your ear, I have sourced some by just listening to air hissing and felt around.

Had this on a 3.2 carrera I was interested in buying. Seller told me it needed 'an adjustment'. I then had a compression test done and found little to no compression and 85% leakdown in 2 cylinders closest to the cat. Thank jesus for PPIs, those were the best $250 I ever spent. I hope this is not the cause of your idle problem.