Engine Diagnostic Help

[SimpleMan]

Hey everyone,
I have a 97 Tahoe LT 4x4 with the stock 5.7 Vortec with 321K miles. Sometimes it has a hard time starting taking up to 4 seconds to start while other times it starts right up and I notice no difference associated with it being hot or cold. It has a slightly rough idle with an occasional backfire but no smoke out of the exhaust. Once I reach a cruising speed of 35 mph to 45 mph, if I accelerate it seems to hesistate unless I press the pedal slightly past half way then it will pick up like normal. If I am trying to get into traffic and the rpm's reach 3000 then after I let off the accelerator it stumbles and runs really rough for about 30 seconds and the CEL flashes, after that it is fine again. I have been having problems with it off and on for a couple of months now and have changed out the alternator, distributor, wires, plugs, knock sensor and harness, ground wires, as well as cleaning the throttle body and egr. I have checked the vaccuum and although I don't remember the specs, I do remember that they were in the middle of the accepted range. Each time I did a repair it seemed to help for a couple of days then it started acting up again. The codes that it throws changes from time to time but the current codes are
STORED
P0102 Mass or Volume Air Flow Circuit Low Input

p0141 O2 Sensor Heater Circuit

p0304 Cylinder 4 Misfire Detected

PENDING
p0141 O2 Sensor Heater Circuit

p0327 Knock Sensor 1 Circuit Low

Today was the first time that I have recieved the Mass Air code and the Cylinder 4 Misfire code although over the past couple months I have recieved codes for Cylinder 1, Cylinder 3, and Multiple Cylinder Misfire. as I stated before I have replaced the knock sensor and wire harness and the voltage has tested within specs for the wire harness. I am at a lose for what to check next and would like to see if you all may be able to steer me in the right direction. Thanks!

 

[SunlitComet]

who's parts and numbers please. you did a compression test? have you tried testing fuel pressure? what are the plugs gapped to? what did the old ones look like, like #4? get started with that and give the maf a cleaning. inspect wiring on your o2 sensors. our the entire engine harness for that matter and the pcn connectors for corrosion. there are also some grounds attached to the thermostat housing and back of right cylinder head that could cause issues as well with what you are going thru.

 

[SimpleMan]

I used AC Delco Pro Iridium plugs with .060 gap, AC Delco wires, Driveworks distributor from advance auto, but later changed the cap and rotor to Accel due to internal cap failure. Ultima Select altenator and BWD knock sensor and harness from O'reilly. The plugs were not erroded but had the white appearance on the electrodes and the one part under the electrodes were darkened. The fuel pressure measures 55 at idle and I have not done a compression test in about 6 months but I believe they ranged from 150 to 160, and I do not have access to a compression gauge so I can't check it right now. I did a visual inspection of the MAF and the screen is completely clear of any debri and dirt and the sensors are clean and shiny. I know the wire on the o2 sensor is bad so that is on my to do list, but would that be the cause of the problems that I'm experiencing? I know it needs to be replaced but I want to work on the main issues if I can find them, because if the engine is shot I'm not as worried about the o2 right now. I have checked all the wire connections that I can and have cleaned them even if they looked clean to ensure good contact. All the grounds are good now as well since I replaced all of the bad grounds, which there was only one fully intact one. The guy that I bought it from butchered a lot of the wiring in it so I made it a point to check all the wiring I could to prevent any issues.

 

[SunlitComet]

do you have a picture of the old spark plugs? have you thought about looking inside the engine wire conduit for damage to the wires in there? I can post some individual test sheets for the codes you listed if you like.

 

[SimpleMan]

I do not have pictures of the old spark plugs but they looked similar to these but the electrodes were in better condition.

As far as the engine wire conduit, are you talking about the plastic covering on the groups of wires? If so I have checked them and they looked good, no melting, exposed wire, or discoloring. If there is another place to check wires I will gladly look. I would appreciate the test sheets you have. Is it possible that the cam shaft could cause these problems since I don't know if it has ever been done and with 321k miles I'm sure some of the internal components are worn. The farthest I have been into this engine is to replace the timing chain so I really don't know the condition of the internals. BTW it still has the original style fuel spider if that helps at all.

 

[SunlitComet]

i would not suspect the camshaft. but if your plugs look like the ones above then you look like you have been burning something other then gas. i am referring to the crud shown on the insulator. did your old ones look like that as well?

 

[SimpleMan]

There wasn't the buildup like the ones in the picture but the insulators were dark brown colored.
If there was anything else burnt it wasn't something I put in the tank, because I only run ethanol free 93 octane and the occasional fuel treatment.

 

[SimpleMan]

Thanks for the test sheets, I have performed several of these tests but there are some on the sheets that I have not tried, and as I mentioned today is the first time I got the MAF and #4 misfire. I'll go over these and see if I can find the problem. It seems like everytime I get a code to go off another comes on.

 

[SimpleMan]

Did a compression test today and the results are
#1 174
#2 174
#3 174
#4 167
#5 179
#6 159
#7 172
#8 171
I know GM says a 20 psi variance is allowed between cylinders and #5 and #6 are exactly 20psi apart. You guys think this is fine or should I test the cylinders further?

 

[SunlitComet]

don't worry about it right now. worry about #4 misfiring at this point.