Check Engine Light.

115

Asked by Nov 03, 2013 at 02:33 PM about the 1989 BMW 3 Series 325i

Question type: Maintenance & Repair

My 1989 325i convertible runs great but idles a little rough. I am planning to replace the injectors; I've been looking at newer Bosch injectors for Mustangs. Is that a good idea?

More to the point though; today I topped off the oil and forgot to put the filler cap back on.  When I stopped at a light the Check Engine light came on and it idle rougher than normal. When I stopped I was lucky enough to find the oil cap. I put it back on and the light never came back on.

Can anyone explain why it would act like that, and can I use that information to improve the idle in normal situations rather than replacing injectors?

4 Answers

254,245

With the oil cap off you created a vacuum leak. No need for new injectors, clean the throttle body, and MAF first. http://forums.bimmerforums.com/forum/showthread.php?875657-The-Full-throttle-body-cleaning-write-up

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115

I cleaned the throttle body and MAF sensor. That didn't work. I replaced the injectors... no help. Mechanic checked ignition on a scope and everything is good. It runs very strong and very smooth. It just has a tiny miss when it's idling that his sensor doesn't even register, but that he and I are both feeling (more so when it's in gear). Any ideas?

115

I cleaned the MAF, and throttle body and tested the idle control valve. My mechanic smoke tested the vacuum lines, and I have no leaks; he tested the MAF, the fuel pressure regulator, and heat sensor and re-tested the ICV. He said the hoses are new and the injection components all fall within a very acceptable range. I did go ahead and replace the injectors with mustang 19# injectors because I read that you could do that without changing the chip. It runs great! But it still misses at idle exactly like it did before. Not a lot; it doesn't even move the tach needle. It's just doesn't fire every once in a while and then the next revolution the cylinder's flooded and it kind of thumps. When you give it gas it feels a little sluggish, but if you put your foot in it, it kind of clears out. Do you think I need an advanced chip with the new injectors? I don't really want to start down the road to custom fuel injection... I was trying to just get the idle smoothed out.

115

bump... ?

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