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Stumble durring initial acceleration.

Tiny
NIGHTSHIFTED
  • MEMBER
  • CHEVROLET
I've got a question about my 1989 Chevrolet K2500 4x4 pickup. Engine is a 5.7L TBI with manual trans with 208000 miles.

The problem I'm having is a single engine stumble durring hard or soft acceleration the first time the truck is moved for the day in first gear. The engine will start off fine then cut out like low fuel then accelerate off again fine. It will do this with a cold engine or after warming the engine. After it stumbles that single time it runs like a champ through all gears both city or highway driving. It will not occour again until the engine is allowd to sit over several hours again. Spark plugs, wires, distributor cap, and rotor have been replaced. TBI has been rebuilt with new regulator and injectors. Fuel filter is also new. Vacuum presure is 21 psi at idle and 15 at highway speeds. Gauge needle stays steady. Spark timing is also right on the money. For a while it was stumbling like this all the time along with back firing, black smoke out exhaust, and code 44 for lean exhaust. I found the egr valve bad and the air managment valve bad putting the engine in open loop stopping the A/F Learned and setting the Integrater to 138 (rich). Repairing that repaired all but the one single stumble I'm getting now. Any Ideas on where I should look next?
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Sunday, December 31st, 2006 AT 11:00 AM

12 Replies

Tiny
KENVCR
  • MEMBER
  • 105 POSTS
I have a suggestion.
Pull out all 8 spark plugs. Keeep them in order as you will be putting these back in.
What do the tips look like? Are all 8 an even colour? Is 1 plug more fouled than the other? Does 1 plug have an oily sheen on it? Recheck the gap to ensure 100% correct.
If yes to any of these questions, you need to have the engine compressioned tested.
This will tell you for sure why it seems to stumble on 1 or 2 cylanders.
You can stop spending money on all the other parts until you know that your engine is "mechanically" sound.
Good luck, let us know what you discover.
My thought is that you may have a worn camshaft lobe or bad valve.
Kenvcr
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Sunday, December 31st, 2006 AT 12:23 PM
Tiny
NIGHTSHIFTED
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When I did the tune up on it all the plugs were the same light brown ashy color. No oil sheen or black carbon on them. I verified the correct gap of the new plugs before I installed them. I thought about a posible valve problem but didn't go any further since the vacuum gauge always stayed steady even through varying loads. The other thing that steered me away from a hard mechanical problem is how intermitant the problem is. Does it just that very one time and then is fine from then on. I'll put a few more miles on the new plugs and then check them again as well as compression. Any other ideas on posible causes would be greatly appreciated also. Thanks for your help. :D
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Sunday, December 31st, 2006 AT 12:32 PM
Tiny
MATHIASO
  • MECHANIC
  • 1,209 POSTS
Did you try to perform a cranking vacuum test.
It used to compare results with the running test. Low vacuum during cranking can indicate external leaks such as broken or disconnected vacum hoses that may not be indicated on running engine vacum test.
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Monday, January 1st, 2007 AT 11:11 AM
Tiny
NIGHTSHIFTED
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  • 12 POSTS
No I didn't do that. Engine starts up fast and idles smooth transisioning from high to low idles smoothly. I definetly will try it though. Thanks for your help :D. This thing is realy starting to perplex me LOL. Thanks again.
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Monday, January 1st, 2007 AT 11:18 AM
Tiny
MATHIASO
  • MECHANIC
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You are welcome :wink:
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Monday, January 1st, 2007 AT 7:48 PM
Tiny
NIGHTSHIFTED
  • MEMBER
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The circumstances of this problem changed yesterday. It will do it now after anytime the engine is started then accelerated through 1st gear. It acts like this. Start engine (fine, idles nice and smooth) begine accelerating in 1st (good till mid acceleration then stumble for a half second then take off fine). Remaing gears all is good. Will run and accelerate normaly all day until engine is shut off and restarted. Then problem will repeat itself. Did get a chance to do a cranking vacuum test. Point test taken at is at the vacuum line for the diverter valve. Line is below throttle plates at throttle body. Disabled ignition so engine wouldn't start and started cranking. To my suprise the gauge read zero vacuum. Can this be right at this check point? Letting the engine start gauge at this spot would read 10psi just as the engine caught and then jump to 22 and 21 while the engine was running.
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Tuesday, January 2nd, 2007 AT 4:22 PM
Tiny
MATHIASO
  • MECHANIC
  • 1,209 POSTS
To perform a cranking vacum test, run the engine untill it reaches normal temperature. Shut the engine off and desable the ignition system.(You did it good),
Connect a vacuum gage to a source of manifold vacuum, then crank the engine several revolutions whiles observing the vacuum gauge. Do not open the trottle plate while cranking.
A steady, even reading of 2.5 in. Hg is considered minimum for must engines.
Uneven or pulsing readings are an indication of defective valves rings, pinstons, or head gasket.
Low readings indicates low compression, incorrect valve timing, the presence of a vacum leak, defective antibackfire on the air injection system, a plugged pcv system, or slow cranking speed.

There are also two additional methods for testing intake manifold leaks.
The first is to pinch off the pvc hose and feel for the prence of vacum at the oil filler, if vacuum is present, the manifold is leaking.
The second is to put propane into the oil filler while observing engine speed, if the speed increases, there is a manifold leak

good luck :)
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Wednesday, January 3rd, 2007 AT 12:49 PM
Tiny
NIGHTSHIFTED
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  • 12 POSTS
What should the normal compression mins and max's be on this engine. It seems to be getting a bit worse lately as the outside temp has been getting below zero. On very cold days when starting the engine cold it has an unstable idle until you give it some throttle. Give it to much and it starts to shake, stumble, and missfire. All you smell is raw fuel. Doesn't take but a minute and she stables out and all is good for it to warm up. Try to drive it before it warms up to about 130 and forget it. When the engine is warm and you take off it still has the other stumble for a quick second like the key was shut off and then comes right back like nothing happened. Engine warm and after that quick cut off in first gear, city driving is great. Get it on the hiway and the light comes on for lean exhaust. Even with the active code I still have pleanty of accelleration. Will take me as fast as I want to go and responds right now. Get back into another town and the engine light will go back out until I hit the hiway again. Fuel presure at idle is at 12.5 psi. I couldn't check it going down the road cause my in line addaptor leaks so I couldn't run it for long.
Thanks for the help so far guys.
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Friday, February 2nd, 2007 AT 5:14 PM
Tiny
2CARPRO JACK
  • MECHANIC
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Just for laughs, unhook and cap the EGR valve and try it, gonna set a code but sometimes you gotta try things, seen TBI trucks pull the EGR on too soon and cause a stumble
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Saturday, February 3rd, 2007 AT 8:58 AM
Tiny
NIGHTSHIFTED
  • MEMBER
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Just went out and tried that for ya. I plugged the vacuum line and no change. Also pulled the cap out of the line while the engine was running and felt no vacuum. I also disconnected it electricaly and same thing. Outside temp right now is 2 degrees and the engine has sat all night, so she's running with the full gammet of problems. Thanks for the idea though. What do ya think I should try next? I'm out of ideas that's why I'm here LOL :D. Thanks alot for your teams help.
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Saturday, February 3rd, 2007 AT 9:28 AM
Tiny
NIGHTSHIFTED
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Well I think today she's finaly had it. It's 13 degrees below zero this morning and I went out to warm it up. It realy didn't want to run unless you feathered the gas. All you smelled was raw fuel and the exhaust was thick and leaving black soot on the snow behind it. Engine had stumbles, misses, and occasional back fires. Feathering the gas you could keep it fairly smooth. Staying off the gass and it would load up and die. Then all of a sudden a large flash from behind and undernieth and KABOOM out the exhaust. Now it won't idle hardly at all. Just misses and backfires no matter what you do. What I would give for a heated garage today. Only supose to get to 2 above zero. I'm thinking that one thing I need to check is the converter even though it's only about 7 months old. With the snow turning black I'm wondering how loaded up the converter is.

Just went out and inspected the damage to the exhaust now that it's light out. Muffler is bent and expanded now but all the pipes are still connected. I guess I'll see what I find when it warms up some and I can take some of it apart.
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Monday, February 5th, 2007 AT 6:26 AM
Tiny
NIGHTSHIFTED
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I cut the exhaust off just before the converter. Tried to run it and it ran just as bad as with the exhaust connected and all I could smell was gas. I pulled the lid off the air cleaner and it was soaked up there even under the lid nut. I got the engine to start and run decent and watched the TB injectors. A few times the engine stalled bringing a big burst of unburnt fuel out of the intake. That explains the gas on top. Once I got it running smooth it appeared that the injectors when the engine was cold were dumping fuel into the manifold. Avery large amount. When the engine started to warm up the spray got lighter and then the left injector apeared to be just barely spraying and right side starting to poor more. Ipulled the assembly off and checked for clogged screens and all looked good. All the passages blew through clean also. I could also blow through the injector screens. I put one of the old injectors back in on the left side and ran the engine again. It's better but it still isn't right. Once I reconnected the exhaust I took it for a ride. Engine hot and at hyway speed got an engine light for lean exhaust. What is making this thing flood out when it is cold and go too lean when it warms up?
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Monday, February 5th, 2007 AT 2:41 PM

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