1998 Tahoe bog down above 50% throttle when cold

[Gordo1728]

98 Tahoe 5.7, upon cold start it will idle fine, on light throttle it will be perfectly normal. Once I pull out of my apartment complex and give it about 50% it will not rev above 2,000, and has no power, after my first acceleration the condition will normally disappear and run like everything is fine. It is not a misfire, I ran miss monitor on my scanner on the drive home and confirmed. It's very comparable to a plugged cat converter, but converter isn't my issue (straight piped from headers to muffler, with proper modifications to get the right o2 sensor readings for post cat sensors). And I performed a head gasket test (chemical) just to rule that out before wasting time. No codes.

Has anyone had a similar condition this? My next place I plan to investigate is fuel system, possibly maf and map sensors. I do occasionally need to let the fuel pump cycle twice before starting, but very rarely. Performing injector clean tomorrow, with injector machine, not seafoam or any cheap additives.

All of the following were replaced
-spark plugs (AC Delco)
-plug wires (AC Delco)
-cap and rotor (AC Delco)
-upper+lower intake gasket (FelPro)
-fuel filter (AC Delco)

 

[drakon543]

Sounds like your fuel pump is on the way out. Its too weak to sustain the engine above a certain point. Ive had this issue once before but on a different make and model.

 

[Gordo1728]

I am really suspecting the fuel pump, have not confirmed pressures yet, gotta get my pressure gauge back from a family member. But don't want to hang a pump if it's not truly my issue. I'll update once pressure is checked

 

[drakon543]

Thats a good start as it definitely sounds like a fuel issue. Suppose it could also be a pressure regulator holding the flow back. It could also be your throttle position sensor they are known to fail in odd ways. It might not be reading past half throttle. That would be a cheap part you could toss at it.

 

[Gordo1728]

Its not a TPS issue, I would have caught it on my scanner. May possibly be a leaky injector. Got a slight fuel smell today, followed the lines from tank to intake, no visible leaks present. But I do believe the previous owner put a junk autozone fuel pump in, i bought a better quality replacement, as well as lines, sitting on the shelf at work. I'm just waiting for it to die completely before I drop that tank.

Still need to hook up for injector service/clean, it's done numbers for a few of my higher mileage vehicles in the past, but I'm not counting on that to take care of the condition.

 

[jerryjoe28]

x2 on the fuel pump. when mine was acting crazy it would be fine at the lower rpms the moment I got in the higher rpms it would just fall on its face. I was hitting right at 55psi at idle. would drop down when I would give it gas. pump as on a year old ( bought from the dealership just my luck lol). new pump and every thing is all better

 

[Gordo1728]

Well when we were doing the fuel injector service I discovered quite a few issues with the wiring to the pump. When we pulled the pump relay, the pump did not shut down. It is wired into another key on power accessory. Why, I couldn't tell you. What disturbed me was the failure to use the heat shrink on the crimp connectors, the second I pulled on the wires they split due to corrosion. Luckily the truck was already set on the hoist at this point. At the top of the image, you'll find the broken wire. The other crimp connectors should never have been used on the exterior of the vehicle.

 

[Gordo1728]

Fuel pump is now properly wired. As for the injector service, wow. What a difference that made in terms of idle and driveability.
The truck was too warm already to tell if the condition was still there.

 

[DallasTahoe]

Fuel pump is now properly wired. As for the injector service, wow. What a difference that made in terms of idle and driveability.
The truck was too warm already to tell if the condition was still there.

What all is involved with a fuel injector service? Did you take it to a shop or did you do it yourself?

 

[Gordo1728]

I did it myself at the shop I work at. In order to do the cleaning service, we hooked to the fuel flush machine up to the shrader valve near the intake, disabled the fuel pump and let the truck idle off of the machine. Once completed, disconnect, replace the pump relay, and it's ready to drive.

I refuse to use most over there counter "mechanic in a can" crap. They most often cause more harm or do nothing at all.