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Erratic cold start RPM fluctuation


IIBRONCOS

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1988 B2 2.9 auto trans. No engine codes and runs excellent when warm. Every time I do a cold start the engine starts very quickly and then immediately starts to jump up and down in RPM's. It will go to 1200 rpm's and then jump back and forth to 500 rpm's real quickly ( engine rocking back and forth ). After about 2 minutes it completely stops and runs great when warm and fully warmed up.
47,000 orig miles and all sensors replaced except IAC (cleaned) and ACT (cleaned). I have also cleaned all grounds I could find.
Any ideas would be appreciated.
 


wildbill23c

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Possibly a vacuum leak somewhere. I have a vacuum leak at the brake booster, must be a Ford problem that's got to be the worst vacuum line connection I've seen. Check for vacuum leaks. A leak can cause erratic idle especially when the engine is trying to warm up. Mine goes up to about 2,000RPM, then slowly drops to about 1,500RPM, then starts going up and down from there, I've actually had it die a few times while trying to warm up from this. I just wait for the RPM to drop the first time then drive away.
 

IIBRONCOS

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I let the B2 sit for a couple hours, and pulled the check valve from the booster and got a whoosh of vacuum. Could this still mean I have a leak there ?
 
wildbill23c

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IIBRONCOS said:
I let the B2 sit for a couple hours, and pulled the check valve from the booster and got a whoosh of vacuum. Could this still mean I have a leak there ?
Click to expand...
Not necessarily there, could be any number of places where vacuum lines are concerned. I was just pointing out where my vacuum leak was for example and nothing to really fix mine that I've found yet.
 

IIBRONCOS

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I just changed the ECT again to be sure it was not the problem.
I still get the erratic warm up idle symptom as before. I have checked for vacuum leaks, and I put a vacuum gauge on the engine and it checks out great doing the various tests with it.
What else could cause this symptom ? I still have the original IAC and the ACT sensors in place. I have removed and cleaned them both. I unplugged the IAC connector with engine running and the idle falls way down like it is supposed to.
 

IIBRONCOS

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FINALLY, a solution for the cold start erratic rpm's wildly fluctuating at cold idle until warmed up. This part that is known to fail when hot, failed right out of the package and gave these symptoms when cold, but no symptoms hot. The ignition control module( ICM ) or thick film ignition TFI module. I was ready to just install a new ECM/PCM computer when I had previously bought an extra ICM to keep in the glove box for a spare. I tried it because I had the extra. Fortunately I did the ICM relocation so it was easy to try the new one. First start up and it smoothly went to 1500 rpm's and gradually came down to the curb idle 850 rpm. It has been a few days and warm up cycles and it has resolved the problem. I was having to go out to and from work and give it 5 minutes of idle time so it would warm up enough to drive. I have had 4 brand new parts installed on the B2 that were defective right out of the box.
 
librum

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wildbill23c said:
I was just pointing out where my vacuum leak was for example and nothing to really fix mine that I've found yet.
Click to expand...
My fix was to use a 'haywire' brand wire clamp maker. Other brandnames.
 
AgPete139

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Congrats on finding the problem. I had this issue on my 92 Mustang, 5.0L.

Something you do do for the future: clean out the vacuum lines anyway. I've found that regular cleanings actually prolong maintenance, like idle and EGR related stuff.
 
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