7000 misfire at idle

[captainruss99]

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

 

[captainruss99]

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.

 

[captainruss99]

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.

 

[74Nitro]

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.

 

[captainruss99]

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.

 

[2015ViperMTX]

Have you tried a different muffler?

 

[2015ViperMTX]

Have you tried a different muffler?

Post a video of the sled running/acting up at idle.

 

[captainruss99]

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.

 

[sxr70001]

Have you looked at your exhaust manifold for cracks?

 

[captainruss99]

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

 

[captainruss99]

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.

 

[Yamasledder]

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.

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.

 

[74Nitro]

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.

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.

 

[captainruss99]

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.

 

[sxr70001]

Bad Injector?