Engine miss / rough running - Where to go next...

[Jehu]

1993 K1500, 5.7L, 5 sp manual,

I've got a couple other threads going but the engine codes are cleared up. Apparently, 42 & 54 can both be caused by loose ground wires at the thermostat housing bolt. Thank you, Richard (someotherguy), for that. The codes are gone but the miss remains.

The idle is a little rough. It's very easy to stall. I need to feather the clutch very carefully to get going. At speed, the miss is evident as well. It's a little jerky with drive train slack.

I've replaced: plugs, wires, cap, rotor, Engine Temp Sensor and MAP sensor.

I have checked the TPS. 0.5v closed. 4.5v wide open. Very smooth needle on the multimeter in between.

What else can I check or "shotgun replace" that can cause wide-range miss like this? From what I understand, the Idle Air Control Valve could cause rough idle but isn't suspect for anything above that. Should I start replacing injectors?

 

[Fuzzy]

I would do a compression test before throwing anything else at it.

 

[Jehu]

I would do a compression test before throwing anything else at it.

There's only 82k miles on it. Are you suggesting valves?

It passed the smog test. They only show the specific results when it fails but the Nevada requirements for a 93 are kind of strict. I figured that would rule out rings and valves.

 

[Ironhead]

I agree with Fuzzy's approach, which is to start with the basics. A compression test will tell you whether the basic top end of the engine (where the fire happens) is in good shape. Having said that, I doubt that the problem is with the valves or rings.

You mentioned a lot of ignition parts have been replaced, but I would also carefully check the magnet in the distributor, as these are known to crack, which can cause firing problems. I would also check the fuel pressure to see that it is in spec.

Let us know how this shakes down.

 

[DerekTheGreat]

I've heard stuff can happen to valves & their seats from sitting. Our '88 454 had 66,*** miles on it when we bought it. Idles rougher than I think it should but cruises just fine. So I went and replaced the distributor with a A/C Delco one, which came with a fresh cap & rotor, A/C Delco wires, the correct copper core plugs and bumped the timing up to 6 degrees from the factory 4. Then before the next oil change I did the water decarbonizing method, probably put two solo cups of water down the manifold with it running at 2k rpm. Didn't help any. That's why now I'd like to do a compression check Our truck also has problems that are most evident when transitioning from open loop to closed loop. It kinda surges and there seems to be dead spots in the go pedal until it's fully warmed up. I've tested my TPS and like you I got the same results. Rebuilt the TB, it helped some. Replaced the upper half of it (regulator & injectors) with a junkyard one and it was even butter but the issues still remain.

 

[DerekTheGreat]

Something else that just hit me and was also told to check, the EGR. I know on Fords it will cause something of a miss at idle if it isn't closing all the way, I imagine it could be the same deal with a GM too. Perhaps that's what my problem is too. Welp, I've got a new TPS coming in along with some other junk like a smog pump and fan clutch as both are ******** the bed. I'll pop that off while I'm there to see if anything shakes loose.