Rough idle and poor fuel economy

[jonathan]

I'm trying to diagnose my newly acquired 535...

Symptoms:

- Rhythmic rough idle - most likely points to a misfire?
- Poor gas mileage compared to what others reported in archive searches - averaged only 18mpg on a recent 350 mile trip - highway miles...

Work I've done thus far:

- Replaced all vacuum hoses - at first I thought the idle was the result of several rotted vacuum hoses. I replaced all of the small vacuum lines and even the crankcase breather hose.

- Replaced spark plugs with NGK ZGR5A - these were the only plugs available locally and being the impatient person I am I replaced the old WR9LS just to see if they would help

- Fuel filter, air filter, all fluids changed

Rather than throw away 90 bucks on new plug wires I wanted to test the resistance first since they don't look incredibly old...couldn't find my damn voltmeter this afternoon! Bentley lists instructions to test the plug wires and the cap + rotor.

I also plan to do a valve adjustment since there seems to be quite a bit of valvetrain noise...I just need to order a valve cover gasket first.

Basically what I need help with is getting together a list of things that I could check. I also read that o2 sensor can cause a rough idle in these cars, and it would make sense that it would also cause bad gas mileage. I'll try to test the functionality with the voltmeter tomorrow. I don't think I want to mess with the AFM adjustment until I rule out all the other things, correct?

Any advice is appreciated - thanks

[shifty]

At what RPM does this rough running happen? I know you mentioned idle, but do you notice a rough, stuttering sensation when going at low RPM in first or second gear? I had a similar problem, and it turned out to be a failed injector o-ring on my #1 fuel injector.

I replaced the o-rings on my fuel injectors, and that eliminated my rough running issues that were similar to yours, my bad gas mileage, and the fuel smell under the hood.

HTH,

[jonathan]

Come to think of it, I do sort of remember a fuel smell under the hood. I'll keep those in mind.

The car runs just fine when not idling. At a light, I can watch the RPM stutter, usually down. The car has never stalled but sometimes it shakes the whole car.

Would those o-rings be available at the local parts store? Did you have to remove the intake manifold to get to all of the injectors?

First gear almost seems non-existent on this car...

[Edit by jonathan on [TIME]1111551644[/TIME]]

[Tjn182]

Shifty --
Would an o-ring leak be detectable by a compression test? How would you find out?

I have some studdering at low RPM -- then again it's probably the fact that I have a turbo and lots of tweaking left to do.... it never ends!!!

[BK]

If the history on your new 535 is sketchy, you can't go wrong with plug/cap/rotor replacement. Wires are probably OK, especially if they ohm out OK and dont create a light show at night when misted with water. Valve adjustment is also in order. Definitely hunt down sources for vacuum leaks.

A new O2 sensor might also be a good idea - a bad one will definitely cause running/mileage issues.

You can check for major intake leaks (like injector orings) by spraying carb cleaner in the area. If the idle speed goes up, you've found a vacuum leak.

If idle fluctuation is the main thing, it might not hurt to inspect/clean the idle control valve.

HTH.

BK

[dworthy]

Here is a common part that tend to fail over time, check your intake boot. Look around the ribs, inbetween the "donuts" and check to make sure the ends are not cracked on the underside. Do test you cap an rotor, and check the oxygen sensor with the multimeter.

[jonathan]

Ok I just checked the plug wires - 6-6.5k resistance across all

Pulled the idle air stabilizer - sprayed the inside with carb cleaner to get all that carbon out. Put some teflon bike chain lubricant inside. The hose that connects to the bottom of this was rock hard and had a small crack at the tip...this was the only crackI could see. What is this hose called?

I was going to check the o2 sensor but didn't have time.
I then took it for a test drive. The idle didn't seem to be any better...if anything it was worse. The idle doesn't flucuate much but the engine shook the whole car so badly I thought it was going to stall eventually. It was raining and damp - doesn't this point to something electrical?

[Edit by jonathan on [TIME]1111609196[/TIME]]


[Edit by jonathan on [TIME]1111609435[/TIME]]

[dworthy]

So the intake boot is good? Do check the oxygen sensor to knock it out as well. The bypass hose can cause some rough idle issues, so do chech both ends for solid non-cracked rubber.

[jonathan]

The intake boot was just fine.

This "bypass hose" coming off the idle air stabilizer was cracked on the only end I could see. I didn't want to remove it since the hose was so brittle.

[shifty]

[QUOTE="Tjn182"]Shifty --
Would an o-ring leak be detectable by a compression test? How would you find out?[/QUOTE]

TJ,

I doubt a fuel injector o-ring leak would be detectable by a compression test. Probably the best way to find a leaking injector is to use your eyes and nose. Shine a flashlight onto each injector -- both where it mates to the intake manifold and where it mates to the fuel rail since there are o-rings in both places -- while the car is idling to see if there is any wetness around them. Use a mirror if you can't see the injectors very well. You can also probably smell the fuel leakage.

I was lucky that my leaking injector o-ring was from the #1 injector. It was very easy to see it when I looked for leakage after smelling fuel under the hood one day.