Misfire Cylinder #5 after warm-up - possible bad spider?

[no cigar]

Hey guys, I am trying to troubleshoot an apparent misfire on cylinder #5 on my 99 2 door Tahoe. The problem only occurs under load, and after the truck has warmed up. It has had the newer style MPI spider installed for almost 5 years now, and the shop near me thinks it's a bad injector. My question is, do those fail at all? I thought they were pretty immune to the same problems as the old spiders.

A little background:

Truck was sitting for almost 10 months getting transmission, transfer case, exhaust work (supply chain issues). Upon getting the truck back going, it took a LOT of cranking to get it running. It did sit the entire time with a little less than half a tank of 93. The battery was out of the truck the entire time. It eventually started, ran like shit and smoked for a while until it finally calmed down when I limped it to the gas station and topped it off with fresh Shell 93, techron, and fuel stabilizer. The truck ran great for almost two weeks until it began to demonstrate a bad misfire. After this, I changed cap, rotor, wires, and all plugs - to no avail. I had a shop look at it and change the distributor. They noted they got it running well again - but after a couple of days the problem was back. I took it back and this time, they mentioned a leaking injector - and want $1200 to do that service. I took a log with a scanner and it looks like the misfires are confined to cylinder #5.

 

[exp500]

Try doing a live data scan, also monitor fuel trims. Try to narrow scope of problem search. A can of berrymans wouldn't hurt either.
Sticky lifters from sitting is possible too, needs a recent oil change. And have valves been adjusted in last 50000 miles?

 

[Eman85]

Did you check or change spark plugs and plug wires? Can you make it miss by power braking? Try power braking, don't go crazy just bring the RPM's up, and have someone mist water from a spray bottle around the plug wires.

 

[no cigar]

I drove the truck today around town and tried to produce various effects at load while monitoring fuel trims. At 75mph/2400-2500 rpm sustained, the stft was great (1-3%). Ltft was not so great however (12-18%). That said I had zero misfires while sustaining speed and only inducing light load to go over overpasses etc. When trying to really give it the beans I could feel stuttering happening but not all the time. It seems to be truly intermittent - but based on load applied.

I haven’t adjusted the valves since the engine was built. But now that you mentioned it, the builder told me a while back that he had to really crank on the rockers just to get to zero lash but said he thought they were good to go.

 

[exp500]

Try a fuel system cleaner again, also watch O2 sensors on live data. From here what you have doesn't seem too bad yet. Have to get closer to determine exactly. Good luck and keep posting!

 

[no cigar]

What would I be watching for with the O2 sensors?

 

[exp500]

If you can graph the scan it will help, save and post here. Want to verify that fuel trims are following O2 outputs. In other words bad O2 vs bad injector. You could also test for leaking injector, but your trims suggest plugged rather than leaking. Again trying to cover multiple bases till focus comes. Every test you can think of will help.
Like Eman85 suggested to eliminate ignition issues.

 

[no cigar]

Just a quick update. This seems to have been fixed with a new thermostat. Operating temps were very low and preventing the truck from fully warming up.

 

[Gebo44]

Just a quick update. This seems to have been fixed with a new thermostat. Operating temps were very low and preventing the truck from fully warming up.

Mine is doing the exact same thing! Except mine(99 tahoe), it’s throwing a misfire on cylinder six. I’ve put new spider (upgrade), new cats, o2 sensors, new plugs, wires, distributor, DS cap and rotor, all new sensors on motor.., including new cam and crank sensors. I did cylinder pressure tests and all was ok. I never thought of the temp being too low! Even though I did tell my wife that the temp guage was always on the same temp and that was odd! It never runs a little warm…, even with 6 inch lift, 35s and mountain roads with hills and alot of pull. I’m going to try a new thermostat and see if it’s a fix!!! How did you figure out to check that? I haven’t seen anything on the net pointing me to it.

 

[m610]

Mine did this on #5, under load. Eventually I started getting exhaust in my coolant. One of those magical fluids you can get at NAPA "repaired" the head gasket, for now, and I have seen the code since.