magarcia00

No engine lights appear and no codes show up. At a speed of 70-74 mph my Tahoe 4WD LT misses badly. If I let off the gas it runs fine and if I floor it the engine performs normal.

If I am on a ramp getting onto the highway and floor it to 95 mph, I have no problem. Only problem is when I creep up to the 70 mph mark.

Vehicle has 135K miles with a rebuilt transmission at 10K miles, fuel pump replaced at 89K miles, platinum AC Delco plugs at 50K miles. Still have original wires cap and rotor. Is this an electrical problem? I replaced the fuel filter a few days ago and ran some injector cleaner thru the tank. problem is very consistent. i turned off the performace chip and the same thing occurs. The analyzer shows no code detected in engine.

mmoss10

I have a 99 with the exact same issue. I have chased this problem for a year. The only thing I've found that made my truck quit acting up at 70mph was Seafoam. I ran two cans through my truck with a tank of premium. I've been running premium gas since then and I haven't had the condition return. By the way, I changed plugs, plug wires, Distributer cap, rotor, EGR, trans flush, U-joints, fuel pump, and, when it first started, I put a set of shocks on it because it shook so bad. I have read every post on this issue for the past six months. I think the most likely root cause of this condition is the carbon build up in the 5.7 vortec engines. Everyone says it's a characteristic of this design. Let me know what you find out on your truck. I hear the Seafoam fix is only temporary. I have also read that a cam position sensor may be a factor, but I never changed mine. It has something to do with the timing. Some also blame it on worn distributer gears. I haven't changed that yet either.

magarcia00

Very interesting! Did you pour a can of SeaFoam right into the tank along with premium fuel or did you pour directly into the throttle body right into the injectors? Last night the Tahoe shook worse then ever at 72mph and so bad that my anti-lock brake light came on and I tried re-setting and getting a code reading directly from the computer using my analyzer and the computer shows no error codes. Everything you say about the distributor gears and cam sensor makes a lot of sense. I need to change my wires and cap/rotor as they are still original. Any chance that this might be a transmission issue while it is under load?? I dont think this is electrical, but mechanical and it is either the transmission, the timing and mix in the carbon build-up issue and we have a real head scratcher going. Have you had the dealer take a look and do you think it is something I should just let them handle?? I am sure they have seen this problem before. Thanks a lot for any comments you have and I will keep you informed.

MT-2500

Get it put on a good engine/transmission capable scanner.
Ther problem should show up and codes should be stored in history or present.
Post back any codes.
MT

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Remember proper testing gives us the answer to many problems.
MT

LastOne

I have had this on my 98 C1500 for a few years now. Thought I had it fixed a few times. Mine is sticking valve guides. Still runs too good to mess with. Shaking is annoying but I just adjust my foot on the accel to get through the moment. I have 110K on my engine. On a recent hwy trip I got 18.7 MPG running at 75 mph. I put 2 cans of Rislone and 3 cans of Mobile Syn oil in last oil change. After 2000 miles it may be just a bit better. My idea is to get a bit of blow-by into the cyl so the intake or exhaust valve stem gets a bit more lube than normal. I have replaced all the ignition system, cam sensor, checked the gear, etc. Chevy dealer confirms the diagnosis but agrees not bad enough to pull the heads. Compression is 185-192 over the 8 cyl. No codes posted on my truck.
Best of luck and Happy New Year,
Steve

16th hippy

i have also seen this when the TCC starts slipping. Mmmoss10 did a tranny flush along with that long list, and that may have been his problem. check your trans fluid for water, or other contaminants and so on. good luck

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Disclaimer: I am not perfect, i may be wrong in my advice from time to time. Also note that any advice given is as is with no warranty whatsoever, and i have zero liability if you get hurt for not being safe while working on your vehicle.

mmoss10

I'm not sure what Hippy is referring to when he says the "TCC" starts slipping. My truck still had the shudder at 70mph even after the trans flush was done (at the dealer). I put two cans of Seafoam in one tank of premium gas and ran it until it was almost empty. That made the condition go away. One more thing my truck was doing was pinging awful bad. I'm still thinking it is the carbon build up.

magarcia00

My friends. Aprreciate all of the comments and it appears there are at least 10 different scenarios for this stalling/missing at 72 mph.

If I can make the condition go away with 2 cans of Sea Foam in the tank I will run over to Pep Boys in the morning and do that. I will also change the plugs and maybe even the rotor and cap as they are all originals. I do have platinum plugs with about 75k miles on them. May check their condition as well. I might dump a can into a 1/4 tank of 89 octane fuel and let the magic begin. I am running a Hypertech performance chip on my Tahoe. Have heard the extra boost can put a strain on misc powertrain components. Any comments. I have run Mobil 1 in engine since new and have run Mobil 1 SHC ATF in Trans after the warranty went out and Mobilube SHC 75W-90 in front and rear diffs.

Still am getting no codes on my analyzer and it is the same as the dealer uses. I think that if my changes don't clear things up, I will have the Chevy dealer give it a shot. If carbon was the issue, why is the performace being affected only at 72 mph.?? I would think that it would run spotty at 90 mph and even idle roughly.???

magarcia00

What is TCC ??

2000CAYukon

Torque Converter Clutch. In 3rd and 4th gear, the PCM will lock up the converter if the throttle position is constant and the speed is the correct range.

//2000CAYukon

LastOne

My truck started out only doing this at highway speed under a slight load or incline. The condition will get worse if it is indeed the sticking valve guides.
My perf is great as noted, 18.7 MPG HWY...
Seafoam used to help but does not help me anymore....
Best of luck.

magarcia00

Greetings...this afternoon took my tahoe into a reputable AAMCO/Cottman tranny shop and this technician jumped on board along with his scanner and test drove the Tahoe as the scanner did its magic. Finall result is that the rpms going into the torque converter from the engine and the output into the transmission is as normal as can be. I was assured that my Tahoe's Transmission is running fine. The TCC is NOT slipping!! I stopped off and picked up platinum plugs, Silicon wires, rotor and cap (ALL AC-DELCO) and if that does not solve the issue I will take it into a dealer. I dumped a can of Sea Foam into the tank with about 5-gallons in it and drove 12 miles to put 10 gallons of premium along with a second can of Sea Spray to see if any carbonization on the injectors gets cleaned up in the mean time.

maxwedge

There is "no pour in the tank" long term fix if the valve guides/carbon is the problem, the guides must be reamed another .0015 to gain the proper valve stem to guide clearances.

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mmoss10

Glad to hear that the tranny is O.K. I think you will see an improvement with the Seafoam. It's freakin' MAGIC. Good luck. If you do decide to take it to the dealer, please post the results! Might save us a buck or 500!

magarcia00

Mr Moss - I will definitely post any and all actions that I take on getting this issue fixed. Tonight I was able to change 8 plugs (kept the original wires, rotor and cap and took the truck out for a spin. The Platinum Bosch plugs were fowled pretty bad and their gap was 0.050 mm. I installed 8 AC Delco Platinum and gapped them to the recommended 0.060 mm.

Still has that severe miss in the mid 60s mph. Tomorrow I will change the Wires, cap and rotor and test drive it again just to see if there is any difference and to see if the Sea Foam has done anything. I cant imagine the valve guides are sticking. I must be living in denial because I just can't see that happening, but there are a couple of guys that swear that is what their issue was. That would be a huge job!! I hope that is not my problem. Will keep you updated!!