5.3 Vortec in K5 - Rough Idle, Stumbles, Running Rich, Runs Like Crap

[PWagon]

I had a pinched O2 sensor wire which gave a bad O2 code. I replaced the O2 sensor and soldered in a new wire section to replace the damaged wire. The check engine light (solid / not blinking) went off and stayed off. All was good.

About this time I noticed a small fuel leak at the metal pipe that connects both fuel rails. I replaced it and its rubber o-rings with a genuine GM part (not cheap). I was careful not to let any dirt/crud into the fuel rail, but a tiny amount may have fallen into a rail while moving the connector out and the new one back in. I put everything back together, and now it idles rough, slightly backfires (pops) at the intake, and runs very sluggish. I took it to a mechanics shop, and they ran every test they could. They did a smoke test for vacuum leaks – no leaks. They did seek error codes P0101 and P0106 which indicates a MAF and/or MAP issue. They replaced both MAF and MAP, and the problem remains. They then opened my up wire harness loom and traced out each MAF and MAP wire back to the PCM to rule out a wiring problem – no wiring problems. An interesting thing they noticed is that the MAP sensor gives a constant reading of 28.3 Hg at all times (when it should be fluctuating). Like I said they replaced it, but no dice. If a small amount of sludge around the fuel rail connector o-ring came loose and fell into the fuel rail while I was replacing it, would it cause this type of behavior? Maybe it’s clogged up some injectors? Any ideas?

 

[broncoman6524]

You 100% sure the injectors are plugged in the correct order?

 

[TheBeast_88K5]

I was thinking bad MAF, or crap in the fuel.
But, I know very little about these engines.

 

[PWagon]

Yes, the engine has run great all year (until now). I have not moved plugs around on the injectors.

 

[dyeager535]

On the older injection (OBD1) you just pulled an injector connector with the engine running and noted if the engine started to run worse. If no change, that cylinder wasn't having issues.

I assume the same would happen on the newer motors, if you can reach the injector plug. You can do the same thing with spark plugs, but I don't like the shock lol.
AFAIK all injectors have a "basket" on the rail side of the injector, as a last ditch effort to keep crud out of the injector, and if large enough to get caught, would probably have to be a significant quantity to actually cause a rough idle, where very little fuel is being used. Again, if an injector problem was an issue though, disconnecting that particular injector should make no difference in how it runs. FWIW, for better or worse, with a somewhat rough idle, it seems pretty hard to detect when an injector is disconnected. The EFI motors (at least mine) seem to run well enough that two or three injectors can be disconnected before idle gets really questionable.

Don't these systems default to either MAP or MAF if either respective component "fails"? Did you try letting it run with one or the other disconnected?

I'm not familiar with these systems, but it would seem odd for an error code to reflect MAF OR MAP problem, since they operate so differently. I would think if anything, the system would bounce the results of one sensor off of the other, and if they don't match what spec is for a given engine condition, the code is thrown.

Assuming the MAF is measuring gms/sec, that measurement at idle is probably a specification that could be checked, then used to focus in on vacuum leak, bad sensor/plug/wire/connector, or whatever. Vacuum leak should show as low gms/sec for MAF, and low MAP, at idle.

If easily accessed, check the plugs to the sensors to make sure contact is being made at every pin (I've had a broken lock on one of the pins, which backed out when it was connected) and if the PCM is easily accessed, I'd pull those connectors off, blow them out good, check the pins on the PCM, and see if the problem resolves itself.

 

[PWagon]

*****Problem Resolved*****

*****Problem Resolved*****
After 7 months of living with this condition, I've found the problem. I only recently noticed that the MAP sensor will send the correct voltage back to the PCM while at the correct angle. Let me explain. I used a brake bleeder vacuum pump to pull a vacuum on the MAP sensor. When vacuum was applied, the sensor is supposed to send less than 5 volts back to the PCM (the more the vacuum the less the volts). Anyhow, I noticed the sensor worked as long as I held it at a certain angle. If I moved the sensor around while testing then the volts would jump back up to 5.2 or therebouts. This told me the damn connector had a back connection in it. So, I went to the GM Stealership and bought a replacement connector, came home, and soldered it in. It now runs like top, and I couldn't be happier. It's truly surprising it's taken me 7 months, and multiple repair shops to figure this problem out. However, it does make me feel good to know that I found the problem instead of someone else. Kinda makes me feel like I gots da smartz. :-P

 

[ktmoutfront]

I have replaced those ends on both of my trucks everywhere on the engine harnes. As they get old, they get brittle.

Any time I find a running issue I start pushing on wire ends first.

 

[PWagon]

It was a hard/painful lesson to learn. But I'll sure remember it.