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7000 misfire at idle

running compression is the same on all 3 cylinders. I will do some more testing this weekend. just tested positive for covid, the hits keep coming
 

I tried another ECU from a friends Viper, same results. Rechecked the throttle body assembly, boots etc.. Bought a new leak down tester, as my old one was kind of crappy. will test tomorrow. I will pull the stub shaft this weekend.
 
Did another leak down test with a new tester, less than 1% leakage, all 3 cyls., when fully warmed up. Pulled the stub shaft, mark is in the correct position, as the dealer said it was. Unhooked the O2 sensor, no change. Will keep going.
 
Is there a place to hook up a carb synchronizing vacuum tool? I realize it fuel injected, but it may help steer you in the right direction.
 
That's a good idea. I don't have one, but know somebody who does. Maybe that can tell me something. I'm running out of things to check.
 
I was talking to a friend with a Viper, and he suggested that I try his gas tank and muffler, for the sake of eliminating parts. I will try this on Saturday. If that fails, I could try to record it.
 
Have you looked at your exhaust manifold for cracks?
 
Only looked at the top, not underneath the exhaust manifold/pipe assembly. I will look under. The tank and muffler made no difference. I have to step back from this for a few days and re-group a bit, before coming up with a new plan
 
I checked the manifold, no cracks. I hooked up an old fashioned vacuum gauge, tee'd into the map hose. I use the gauge all the time on automotive engines to diagnose internal issues. A correct automotive engine will run about 18" of vacuum, steady needle. This one is bouncing quite a bit between 5 to 10". If this was a car, I'd say we have some internal issues to look into. I have never hooked this up on a small engine, and have no experience to fall back on. I assume it would be similar to a car engine, but don't know for sure. I lost the test Viper I had at home, as it went to the dealer as a trade in. By the way, the timing marks on the cam sprockets are correct, as I didn't mention that earlier. I will have to do some research about the vacuum readings.
 
captainruss99 said:
I checked the manifold, no cracks. I hooked up an old fashioned vacuum gauge, tee'd into the map hose. I use the gauge all the time on automotive engines to diagnose internal issues. A correct automotive engine will run about 18" of vacuum, steady needle. This one is bouncing quite a bit between 5 to 10". If this was a car, I'd say we have some internal issues to look into. I have never hooked this up on a small engine, and have no experience to fall back on. I assume it would be similar to a car engine, but don't know for sure. I lost the test Viper I had at home, as it went to the dealer as a trade in. By the way, the timing marks on the cam sprockets are correct, as I didn't mention that earlier. I will have to do some research about the vacuum readings.
Click to expand...
Because this is a 3 cyl. engine, there's are long duration between cylinder cycles and will give you a pulsating reading on the vacuum gauge. The way to prevent this on this engine is to put a restrictor in the hose. I just pinch a piece of copper tubing until it's almost pinched closed and then put it in the hose. I think you'll see a difference. The reading will be higher and steady.
 
captainruss99 said:
I checked the manifold, no cracks. I hooked up an old fashioned vacuum gauge, tee'd into the map hose. I use the gauge all the time on automotive engines to diagnose internal issues. A correct automotive engine will run about 18" of vacuum, steady needle. This one is bouncing quite a bit between 5 to 10". If this was a car, I'd say we have some internal issues to look into. I have never hooked this up on a small engine, and have no experience to fall back on. I assume it would be similar to a car engine, but don't know for sure. I lost the test Viper I had at home, as it went to the dealer as a trade in. By the way, the timing marks on the cam sprockets are correct, as I didn't mention that earlier. I will have to do some research about the vacuum readings.
Click to expand...
These engines make max power in the 8000's rpm range where a car is substantially lower. They are able to make power at high rpm's due to aggressive cam profiles that give up low end power, and thus create lower idle vacuum.
 
Thanks for the info, guys. I have 46 years in the automotive field, but am a bit weak on smaller engine diagnostics. I will check this out a bit further. I may have a test Viper from another friend to compare to. The thing is, when it's not misfiring it runs and sounds normal. It's not a constant thing.
 
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