Author Topic: Rough idle/take off (Read 4671 times)

tumbler · « **on:** May 23, 2009, 02:37:20 PM »

I've got a rough idle problem with my '93 318i that has me a bit stumped. I've checked the vacuum hoses, TPS, coil packs and fuel pressure. Installed new O2 sensor, new crank/cam sensors and plugs and I've cleaned the MAF.

Its tripping the CEL, 1222 (Lambda) code.

Here are my symptoms:

- Rough, choppy idle that persists until engine RPM's get up around 3k. If I really slip the clutch/keep RPMs up the car won't stumble.

- on take off it feels like it wants to die but never does (If I don't really slip the clutch/keep rpm's up out of the hole)

- when idling the problem comes and goes in a cycling manner. If I listen to the exhaust I can hear a "put, put" noise when the idle gets rough, leads me to believe its missing on a cylinder.

- Problem is not noticeable if the car has sat for a few days; however it will return after 20-30 minutes or drive time.

- The problem gets much worse if I have my AC on.

- Exhaust does not smell rich

derek9702 · « **Reply #1 on:** May 23, 2009, 05:23:12 PM »

If you've checked for vacum leaks already, I would check/clean the ICV valve.

tumbler · « **Reply #2 on:** May 24, 2009, 06:44:12 PM »

Whats the best way to check for vacuum leaks? I forgot to mention that I've cleaned my ICV before as well. I just looked at any visible tubes for cracks and I replaced one skeptical one.

DesktopDave · « **Reply #3 on:** May 24, 2009, 07:45:06 PM »

That's an odd one...my guess is throttle position sensor or AFM. The TPS is easy to get to, I'd go for that first. There was a great thread on R&R recently:
http://www.m42club.com/forums/showthread.php?t=7553&highlight=throttle+position+sensor

That O2 code means that the car is running rich or lean, not necessarily a bad sensor. That's why I'm thinking that the DME is doing this on purpose with a bad sensor reading. Sometimes they only fail with a little heat or vibration, testing just fine when they're cold.

Did you check the spark plug leads?

I've seen techs use WD40 to squirt on vac hoses to test for leaks. I've never had any luck with that, just replaced everything that even looked like it might crack.

tumbler · « **Reply #4 on:** May 25, 2009, 09:53:48 AM »

Alright guys. I'm slightly embarrassed that I didn't notice this before, but the vacuum tube leading form the fuel pressure regulator was badly damaged and on top of that didn't fit tightly at either end. It was kind of hidden under the TPS so I didn't notice it before. I haven't been able to fully test it to see if I've fixed it.

What is the function of this vacuum line?

DesktopDave · « **Reply #5 on:** May 25, 2009, 11:24:25 AM »

I always thought that the vacuum line pulls against an internal spring to raise fuel pressure as vacuum decreases. So when vacuum is high at idle, you get low pressure. When vacuum drops at higher RPMs, the regulator feeds more pressure to the fuel rail. That keeps the relationship between intake air vacuum and fuel pressure constant:
http://www.mirafiori.com/~thad/fi/fpress.html

Glad you found a problem, that'd definitely make for rich and lean areas that the DME can't compensate for. Let's hope that's the whole enchilada!

I wouldn't be embarrassed at all (unless the problem went on for like fifty posts), it's the ideas that count. I check mostly everything that gets mentioned on this board on my own car - it's saved me a lot of time & frustration over the miles.

tumbler · « **Reply #6 on:** May 27, 2009, 12:29:22 AM »

Update:

The car drives exactly the same as before. Its gets VERY rough when I run the AC, which is esentially a must with a black on black car in AZ. Leads me to believe the fuel regulator is bad if it doesn't make a difference with the vacuum tube on or off. Thoughts?

DesktopDave · « **Reply #7 on:** May 27, 2009, 06:52:52 AM »

I wouldn't condemn the FPR right away, I'd test pressure and flow. Might be a bad pump, could be a different vacuum leak. Does Advance Auto rent fuel pressure testers? I've used them a lot of times for things like spring compressors & brake tools. Maybe pick up a cheap vacuum gauge too, run some tests.
Does this rough running happen all the time, or only when the engine is warm/cold? The computer should raise the idle for A/C.

tumbler · « **Reply #8 on:** May 27, 2009, 08:38:07 AM »

Quote from: DesktopDave;72466

I wouldn't condemn the FPR right away, I'd test pressure and flow. Might be a bad pump, could be a different vacuum leak. Does Advance Auto rent fuel pressure testers? I've used them a lot of times for things like spring compressors & brake tools. Maybe pick up a cheap vacuum gauge too, run some tests.
Does this rough running happen all the time, or only when the engine is warm/cold? The computer should raise the idle for A/C.

It generally runs rough all the time, although the car does much better if its been sitting for a day or two. I have a fuel pressure guage, when i have time I'll run some tests at idle, around 3k and then what happens when I bring it back down.

the idle does not pick up when I turn on AC. The car idles around 6-7 hundred all the time, it gets worse when it cycles through those rough spots while sitting at idle.

tumbler · « **Reply #9 on:** May 31, 2009, 06:05:10 PM »

I tested the TPS again. I got 3.81k ohms with the key off and the throttle closed (idle position). When I move the throttle the ohms stay steady all the way to WOT. The ohms never move off of 3.81k...seems to be the throttle closed reading is slightly low and the throttle test definetely failed.

DesktopDave · « **Reply #10 on:** June 01, 2009, 01:31:12 PM »

twright did a great tutorial on R&R'ing his TPS:
http://www.m42club.com/forums/showthread.php?t=7553&highlight=throttle+position

Might not work, but the price is right!