88 Carrera - rough idle

[Raulg]

I have a 1988 Carrera with an intermittent condition.
Some times it will idle rough and if the car is coasting, it feels as if it wants to stall.
Most of the time it idles fine.
Could this be related to the fuel filter or pump?
I have checked all the ground connections I could find and cleaned them, including the ground straps under the car.
I recently replaced the ignition wires and it appeared to help by the hesitation has come back.
Please help.
Thank you
Raulg

[Hunt3R]

When it idles "rough" what RPM is it idling at? 700? 600? Does it stay there or keep dropping? It only happens when you're moving and take the car out of gear while slowing down to a stop? And not everytime?

[Storage Man]

When was the last time your fuel filter was changed? Injectors cleaned?CO checked?

[kidrock]

dirty ICV? Next time the intermittent issue pops up, lightly tap on the ICV with a screwdriver handle. If you notice a difference, you have found your issue. Very simple to R&R and clean.

[Raulg]

To all,
Thank you for the responses.
1. The car idles rough at 1000 rpm or so. When it is standing still. When it coasts down in gear at say 40-45 mph will do too.
2. Fuel filter, injectors not touched in the last 5 years or so.
3. Where is the ICV? Perhaps it is right in front of me, I just don't know where to look for it.
I appreciate the help.
R.

[rsscotty]

The valve is on top of the engine, forward of the fan housing.

[ozracer]

How do you replace or service them does that mean you can pull them apart and clean them up.

[steely]

You can remove it to clean it - but I wouldn't pull it apart.
ICV removal & cleaning

@Raulg: I would also suggest that you check the idle position switch to see if it is working - located on the drivers side of the throttle body (TB) - this tells the DME when to activate the ICV to maintain idle. The ICV only works at idle.

From a cold start, the DME uses the ICV to maintain idle a bit higher than normal - as the car warms, the idle is lowered. If you have a rough idle only when warm, this is a good clue that it may not be the ICV (or the switch) but you can check them.

Other culprits could be those mentioned above, or a even something simple like an air leak, so check all of the rubber vacuum lines large and small to make sure they're connected and in good shape (not cracked). The ICV may be trying to compensate for a leak. Also check the intake to make sure the nuts are tight. Resist messing with the TB adjustment screws.

Sometimes if a PO had a small leak (which would lower the idle), they may have messed with the adjustment in order to raise it back up. If this case if you found and corrected an air leak, you would know it because your idle would increase. Some will spray oxygen or carb cleaner judiciously (flammable) around the intake to help find a leak. Another clue reveal-er would be to remove the oil cap once the engine is warm and running to see if you get the normal decrease in idle speed (after about 20 secs) - if it doesn't decrease, you have an air leak.

There are a few other things you can do of course, but I would recommend getting a Bentley manual to refer to, or even searching around here for some advice (granted - not easy if you're new and not sure what to look for). Keep us posted.

[steely]

[Tippy]

Sounds like a vacuum leak test is needed. 1000RPM idle means 1 of 2 things:

1. A vacuum leak is causing higher idle
2. Someone raised idle tampering with the various idle adjustments to mask a vacuum leak

There are lots of places vacuum leaks can pop up. Especially concentrate on intake to head area near fan shroud. There's 2 gaskets and an insulator that are frequent to deteriorate per cylinder/runner.

There's several vacuum hoses attached too that have been problematic with age.

[rick-l]

If the idle is really 1000 rpm warm you need to look at the temperature sensor.

[Raulg]

Steely and others,
Thank you for your reply. I will take the ICV out and clean it.
The engine idles at about 650 to 700 RPM but it drops and bounces back up 50% of the time.
I noticed that when checking the oil, the engine RPM drops, so hopefully no vacuum leaks. The large rubber hose that connect to the oil filler tube started to crack but no leaks are present that I can tell.
I will report back.
Thank you for your help
R.

[kidrock]

if you haven't replaced your fuel filter for 5 years, I would recommend doing so.

[rick-l]

Idle on a warm DME car according to the Bentley is 800 RPM +/- 20 and is not adjustable with out a EEPROM reprogram.

Are you rounding this indication on the tach up to 1000?

Would not hurt to stick an ohmmeter on your CHT when warm. It is on the bracket with the speed and reference sensor connectors.

Wouldn't leaks past the metering device cause the rpm to drop?

[steely]

Raul - I went back and read your first post and now realize you said intermittent. (sorry - it was 2 AM for this insomniac)

So, not to go completely into punt mode on you, I do question whether my suggestion of an air leak would act like that - they're not usually intermittent.

If you could characterize the rough idle as rough or bouncing, verify the idle rpm, and the rpm when coasting in gear when this happens that might help others guide you.

Also, besides the CHT measurement rick-l suggested, maybe start the car when cold and let us know the rpm and see if it drops as the car warms - one symptom of an open/ bad CHT would be that the starting idle never lowers. If the CHT is intermittent, you may not see a problem when measuring it, or when starting the car, but it could act up and bug you as you drive (not just in idle).

DME/motronic problems are hard (but fun) for me to diagnose, so I don't want to lead you on with any guesses on my part. Changing out old dried out vacuum lines is cheap and never hurts either, but might not be the primary issue here. If the ICV is sticky, that still isn't a bad idea.