Diagnose This If You Can

[only126db]

Parents have a 1999 Tahoe 5.7 4x4 in excellent condition with around 130k miles.

Mother said her truck would feel like it would lose power when going up a hill.

The tune up was long past due so they figured it was a faulty wire or something.

They got the tune up done which consisted of plugs, wires, cap, rotor and fuel filter.

The issue was still there, then there was a comment made by them about a vibration sometimes but they could not get it to repeat consistently.

The shop took it for a ride and was able to get it to repeat readily.

There was a miss in cylinder 5 and a couple others and throwing a general misfire code.

They went to pull the new plug wires off and one fell apart so they replaced them with an upgraded set, and on the scope there was a misfire on cyl 3 around 5x and on cyl 4 over 100 times.

Plugs were switched around and same issue with #4

Disconnected certain sensors, egr etc to see if it made a difference and no.

There are no codes being thrown, no dash warning lights.

I did a SeaFoam intake clean out just for sh*ts and giggles, it blew a ton of smoke, more than any of my vehicles have and I have beaters.

I took it for a ride today to see exactly what it does, and it actually feels more like an out of balance tire, there was no odd engine sounds, no nock, no ping, nothing, it felt like a flutter.

This happened at 48 mph in overdrive in top gear and just giving it a little gas to keep speed at that level and not have it downshift.

It did it and progressively got worse then stopped, but we were at the top of the hill (we have one small hill to test on).

I tried a couple more times but not in overdrive to see if it would do it when not in overdrive, i could not duplicate it, but this could be coincidence.

I was told (all guesses) it could be distributor, injector spider, an injector, torque converter and valves sticking.

Also note it idles smooth, hammer the throttle and no issue, drive around and nothing, smooth as silk, just under a load like a hill (at speed stated) or towing a trailer(wasnt there with trailer dont know circumstances).

What do you think???

[matthew24u]

good luck it only acts up under a load could be anything. so after all you have done you at a good starting point next get a hold of a scan tool and look at datastream what are the o2s doing first if there working fine just look at all else and make sure they are performing within specs that is were i would look next and i didn`t read you saying that you have replaced the fuel filter. how does that look? signed always stuck in diagnostics!

[grog11]

Have you thought about having the battery and alternator load tested to see if either is weak? You could remove the battery terminals to reset and relearn the computer functions. You could also check all the ground straps for continuity at the terminals. Grounds can do funny things intermittently with the ECU modulation of transmission solenoids and sensors.

No code engine issues are the hardest to determine a problem from.

Fuel pumps tend to fail around your mileage with the Tahoe. Not sure what your fuel pressure psi is supposed to be.

[only126db]

Originally Posted by matthew24u

good luck it only acts up under a load could be anything. so after all you have done you at a good starting point next get a hold of a scan tool and look at datastream what are the o2s doing first if there working fine just look at all else and make sure they are performing within specs that is were i would look next and i didn`t read you saying that you have replaced the fuel filter. how does that look? signed always stuck in diagnostics!

yes it was replaced, i didnt see it but from what i was told it was blackness inside.

Originally Posted by grog11

Have you thought about having the battery and alternator load tested to see if either is weak? You could remove the battery terminals to reset and relearn the computer functions. You could also check all the ground straps for continuity at the terminals. Grounds can do funny things intermittently with the ECU modulation of transmission solenoids and sensors.

No code engine issues are the hardest to determine a problem from.

Fuel pumps tend to fail around your mileage with the Tahoe. Not sure what your fuel pressure psi is supposed to be.

Battery is brand new, fuel pressure was in the upper 50's dont remember exact number though. I keep seeing different specs stated on this too, anywhere from 50-60, I think it was 54, but like i said not 100% sure what it was.

I went back to the one shop and the guy told me it needs head work because its common for the valve stem to be to tight in the guide and thats the issue,(but why would that only make an issue at a certain speed) then he also said the injector spider will need to be replaced, sounds a lot like someone not knowing whats wrong and trying to make some cash.....

[777stickman]

Worn Dist gears are an issue. To check, pull the cap and hold slight down pressure on the rotor. Gently turn the rotor. There is no rotational play allowed.

Fuel press needs to be 60-66 psi with Key on Engine off and needs to hold 55-60 psi when the pump stops.

Not being able to remember exact readings does not help us much!!

[matthew24u]

when you get hold of that scantool you are going to need to complete your diagnostics after rereading your symptoms at a certain speed ,you may want to look for a code in transmission. some scantools it is a seperate screen you have to go into to read the trans codes. but ithink you will see what your looking for in datastream or at least get closer to diagnosis . oh and definetly stay away from that shop who wants some- Head.
cause i think your going to get f--ked. let me know what ya see in datastream.

[tahoedriver]

A year back I had my 97' 5.7L tahoe throw some codes. I first noticed it pulling a U-haul on I-40 coming home from college. It would lose power and I would feel a very slight spudder. The codes it threw stated misfire in cyclinders 3 & 5. Also, it would hesitate on start-up. In the end, I switched out the spider injectors (after doing research on the topic and hearing advice) and gave it a full tune-up. Problems solved. No more codes, power back (power for an engine with 227,350 mi on it), and no more ingnition hesitation. The injectors are prone to failure. The poppets become clogged which can wreak havoc. The new assembly is a much needed upgrade, I would suggest. Sorry for the short story but that was my previous experience with loss power and cyclinder mishaps, hope things work out. p/s, bad distbributor gears are also something to look at as mentioned before.

[danielsatur]

Random misfires on random cylinders!
Fuel pressure test
New Air filter?
Bad or dirty MAP sensor
Fuel/Air - Throttle body fuel meter
Catalytic converter back pressure test