Rough Idle

[cutsham]

Just bought the car yesterday: http://www.r3vlimited.com/board/showthread.php?t=270998

Just shy of 98,000 miles. Idle gets a little shaky, it will be at 1000 rpm, but fluctuates between under 800, and a little over 1000. Also, a quick pump of the gas causes the engine to bog down a little. Sounds like its about to die at times, but never has. However, when driving it runs really smooth, and is very responsive.

Previous owner thought it had a vacuum leak, and originally thought a new oil pan was needed but then wasn't so sure.

Borrowed a mityvac vacuum tester today


Hooked it up to the intake manifold, and at idle the vacuum was a steady
13 Hg. When hooked up the idle remained choppy.

Then I noticed my valve cover:


Obviously the gasket needs to be replaced, but should this fix my idle and low vacuum?

[jlevie]

Have a shop run a smoke test. That will tell if if there are intake leaks (very likely) and where they are.

[dnguyen1963]

You have found at least one of your vacuum leaks...smoke test will find the rest.

[cutsham]

Have a shop run a smoke test. That will tell if if there are intake leaks (very likely) and where they are.

You have found at least one of your vacuum leaks...smoke test will find the rest.

How much should a smoke test run?

[dnguyen1963]

About $100 at a shop. Dealer will cost you a lot more. You can also make your own smoke machine...search the net...but remember to keep the back pressure at 2-4 psi (I have used a simple aquarium pump) to fine all the leaks. Here is a brief description of how to cheaply perform a smoke test.

1. Buy a pack of cheap cigar from Walmart ($5)
2. Crush the cigar and stuff it into a copper pipe
3. Connect the copper pipe to the inlet of the pump (put a cotton ball inline to trap smoke particles)
4. Light the cigar then cycle the pump on/off (mimic smoking action) until you get sufficient amount of smoke coming out.
5. Cover the exhaust
6. Connect the pump outlet to the vacuum port used for the FPR
7. Cycle the pump on/off for about 5 min to get enough smoke into the engine
8. Keep the pump on constantly to achieve 2-4 psi back pressure
9. Look for smoke leaks around your engine.

This method works if you like tinkering with making your own device. It does require some optimization of the setup to get sufficient smoke into the engine, but once optimized you have your own smoke machine.

Good luck.

[cutsham]

I still wonder if there even is a vacuum leak. It idles fine, and every so often during idle it sounds like its about to die, but never does, then runs fine again. Also, the gas milage is really bad. PO thought he was getting between 12 and 14 mpg.

[dnguyen1963]

Check the elbow connecting the IVC and TB. It is a frequent source of vacuum leak, but you should still perform a smoke test any time you have irregular idle.

[Aleman]

The Bentley manual shows how to test the coil. It's worth a try.

[cutsham]

Thinking I don't have a vacuum leak after all.

Messed with the Idle Air Stabilizer Valve (ICV). Mine is the Motronic, with three pins. Tested for resistance which was good, cleaned, and functions and closes when connected to power. However, when the engine is running and I unplug the wire harness, the idle does not change whatsoever, or rev up to 2,000rpm like it is supposed to when unplugged.

Could my problem be a bad DME?