360 won't idle smooth...can't figure why

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cudamark

cudamark

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Hi gang, normally people are asking me these questions but this one has me stumped. Hopefully someone here has run across this problem. Vehicle is a '91 W250 automatic with the above mentioned 360 throttle body injected. Engine has good compression and tuneup parts but the RH inner two cylinders (4 and 6) aren't doing anything and the LH outers (1 and 7)aren't pulling their weight either. You can pull off both inner plug wires and there is no change in RPM. Throttle body is spitting normally (in fact, I swapped another one onto to it just to be sure and there was no change), EGR is working good with good gasket, PVC valve and system is working properly, Valves are going up and down normally, and no vacuum leaks that I can find. Dribbling a little fuel out of a primer can doesn't improve the idle so that confirms to me that there is no vacuum leak. I haven't had it on a smog machine yet to see what the readings are so no input there. Since there is only one O2 sensor (in the LH exhaust manifold) I figured that wouldn't be the problem since it can't be cylinder selective. The truck stays cool and runs fine out on the road and is not sluggish on acceleration. I'm kinda thinking that the exhaust crossover on the RH side is cracked and leaking into the intake runner on those two cylinders. Has anyone seen a throttle body intake crack like that? It's a bit of a job to pull and change with all the plumbing and being 5 feet at the top of the fenders so I'm hoping it's something else. Even if I had it off, there isn't an easy way to tell if it's cracked since that area is buried inside. It's a dual plane intake so you can't see the area in question from the top as that section is the lower level and under the upper level. If it's cracked, what would be different on the smog readings over one that isn't cracked? I'd hate to go to the trouble of changing it and find that wasn't the problem. If anyone has had a carb engine with one side of the idle circuit plugged up, that is the closest symptom to match this problem....acts like it's idling on 4+ cylinders. Any ideas would be appreciated.
Thanks, Mark
 
I think you could have a few issues at this point. Check the coolant temp sensor that the ecm reads reads, not the one for the gauge. See if it corresponds with the temp gauge. You will need some sort of scanner to monitor it or remove it and ohm it out. It may be bad and running the motor full rich. Also I wouldn't rule out the 02 sensor, its a good possibility i could be bad as well.
 
The plugs appear to be burning clean and there is no soot in the exhaust so it doesn't seem to be running rich. It runs the same cold or hot. How would a temp or O2 sensor make just those cylinders not work? wouldn't it effect them all? I've added extra fuel and opened a vacuum hose to make it run leaner but neither will smooth out the idle....even for an instant.... before the computer compensates.
 
Maybe burnt valves? I have also seen the pick-up module in the distributor cause strange things. If its not running rich that would rule out the sensors I mentioned. I would run a compression and leak down test to verify the engine itself is ok.
 
Sorry in advance if this is something you've already checked - plug wires? Maybe grounding at idle and free when the engine picks up? Could a bad ECM drop the spark on those 4 cyls? I'd just sure it is not a spark problem before I pulled the intake.
 
I had a similar thing after the first few months of the 360 being in my Barracuda. It would idle ruff but run OK down the road. Could pull of plug wires and there would be no change in rpm and my compression on all cylinders was/is 195 +/- 5 psi.

It turned out to be the ignition wires. My daughter had given me a set of the cheap yellow wires that only had the carbon loaded conductor (no wire over wrap). They even measured in spec with an ohm meter. In despiration I swapped to a set Moroso Blue Max spiral core wires I had and low and behold the rough idle smoothed right out.
 
I believe that the clue here is the cylinders involved and the fact that the intake is a dual plane, I had a similar problem with a 318 and a factory 4bbl intake where the casting between the paired ports had some flaws and the gasket wasn't sealing properly. This caused the paired cylinders to be lean at idle. There was no missing under load or at cruise and the plugs looked normal. I replaced the intake and no more problem.
 
BadAss71 said:
Maybe burnt valves? I have also seen the pick-up module in the distributor cause strange things. If its not running rich that would rule out the sensors I mentioned. I would run a compression and leak down test to verify the engine itself is ok.
Click to expand...
The first thing I checked was the compression and it's 140PSI across the board so I figure there are no burned valves. I get a good, regular spark right at the plugs so don't think ignition is to blame. Irazor's problem seems the closest to mine but I would think that if there was a vacuum leak or lean condition of any kind, adding a little raw fuel down the intake should smooth it out wouldn't it? I'm still thinking the intake is cracked between the exhaust crossover passage and the intake runner to those two cylinders. I've just never seen it happen on that particular intake before. That would effect the fuel charge to those two cyinders and may effect the other two on that plane as well. The only thing keeping me from deciding "That's it!" is: time and effort for one....and somehow I would think I would get a backfire if the fuel was being burned in the intake runner before it got to the valves for those two cylinders. Maybe the timing is just right to prevent that but it just bothers me and gives me enough doubt that I may delay making a final decision until I get more info. Does anyone know what the smog reading would look like if exhaust gasses were getting mixed with the intake charge? Like an engine with a leaky or stuck EGR valve?
 
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