89 IROC 5.0L/M5 car originally replaced with GMPP 350/330 crate engine. TunedPerformance burnt the chip for me and its been running great since I put it together last summer. The car is my son-in-laws and while visiting this past week (about 1,000 mikes away from my home), he's been observing a vibration that's getting worse. Soon as I started the engine I felt it missing and am assuming this is the vibration he felt. Its noticeable at idle, cold or warm, and definitely gets worse as rpm goes up. It smells funny and can hear popping once in a while through the exhaust. Slow to rev.
With minimal tools available and even though all these parts are a year old, I just threw the book at it ... plugs, wires, cap, rotor, fuel filter. All spark plugs looked the same. Light tan. No change with anything.
Run to Autozone and rented a fuel pressure gauge and a noid light for the injectors. About 40psi and all the injector connectors showed a pulsing light.
I'll get back with my timing light, laptop and TunerPro in some days. So that's the background on my trip and trying to think what could cause these issues. Anyone ever experienced bad gas and had these symptoms?

 

Throw a bottle of Royal Purple fuel injector cleaner in the tank and run it thru. Could be as easy as a dirty injector

 

Thanks for the reply. I pulled each injector connector and tested with a plug in noid light. I know it doesn't confirm the injector itself is firing, but at least I know the pulse is getting there. Each connector I pulled made very little difference in the engine running. I feel whatever is happening is affecting the whole engine, not just a cylinder.
The injectors are new Bosch installed with the engine last year.

 

New injectors doesn't mean anything tho. I had brand new Holley 120# injectors that were dirty right out of the package. A flow test showed that 3 were down about 8%. Once cleaned they were all 1% of each other and flowed considerably more than 120#

 

If there's a noticeable miss at idle, you can go around the engine unplugging one injector at a time. When you encounter one where the idle doesn't get any worse, that could be a faulty injector.

 

However, I'd add to that, by suggesting that before doing that test, disconnect the IAC so it doesn't correct idle speed and cause....

Also, you can "shoot" the header tubes near the head to verify they're all contributing about the same amount, based on temps.



At this point though, (3 days or more), you could have simply emptied the fuel tank and filled w/different gas to eliminate that, as the cause. I've never gotten "bad gas" from a gas station and I've never had gas "go bad" to where it caused drivability issues....even 5 year old gas in a sh!tbox that I bought, still ran and drove totally fine...though it did reek.

 

Agreed, but since these ran fine when new and just started having this issue in the past couple weeks doesn't sound like dirty injectors to me. The tank was dropped and cleaned and new pump/sock and filter as well.

That's certainly a strange part to this. There was hardly any change when I pulled Inj connectors one at a time. All 8.
No headers. Owner wanted stock exhaust other than a catback.
Yeah I may have to siphon it dry when I get back. I didn't have time to do while I was there.

Thanks for all the suggestions guys

 

Update and another life lesson learned... When things don't make sense, go back and recheck your work!

I made the trip back with TP and my laptop. Live data says lean. INT/BLMs up in the 150-160 range and just bouncing all over the place. O2 doesn't seem very responsive. Half-filled a Qt glass jar from the fuel rail. No noticeable contaminants or water in the fuel. Pulled the injectors to attempt home cleaning and look for an obvious problem. Thought we found one having an issue so in with a new set. No change. Still missing bad. Still showing lean. Couple people suggested compression test. Pulling the new plugs back out a second time revealed a clue. After all the testing, idling several minutes etc..one plug showed an issue. one cyl was fuel fouled. How could one cyl be fouled with new plugs, wires, cap, rotor and even new injectors!?!? After several emails back and forth with the ever helpful Brian @ Tuned Performance, he said "that just doesn't make sense". It didn't make sense, so I replaced the plug wire with another new one and immediate change. BLMs back to normal. No more miss.
Unbelievable.
I am assuming the ECM was showing a lean condition because the o2 was reading extra oxygen in the exhaust due to the unburned mixture from this cylinder. It was on the drivers side.
Maybe posting this situation will help someone else down the road..

 

That's wild. So what the spark plug wire was broken on the inside then?? I don't think I've ever heard of something like that

 

I didn't disect it but I believe it was intermittent enough to cause this. I could see it fire with the timing light which was also throwing me off.

 

I had that happen on my 'Vette a few years ago. It busted my ***** for a minute, then I thought to myself, "Something is going on here...." and I went after the wires. One ohmed infinity. That was the problem. Never had seen that happen before.