1994 Chevrolet Silverado mystery problem please help!

Tiny
THOMAS WAGNER
  • MEMBER
  • 1994 CHEVROLET SILVERADO
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 180,000 MILES
I recently purchased this truck and when I did it was running perfectly. The next day it began giving me issues with a few second delay in time from when the gas pedal is depressed until the time when it actually throatled forward. Anywhere from half second to 3 second delay time there.

I changed the fuel pump, fuel filter, spark plugs, distributor cap, rotor, spark plug wires, then timed it; ( it was about 10deg off) fixed a few small vaccume leaks. These repairs were done during the day time at 2 separate times, both of wich by the end of the day; THE TRUCK BEGAN RUNNING PERFECTLY.

UNTIL THE NEXT MORNING WHEN THE TRUCK WAS COLD! IT THEN BEGAN DOIGN THE EXACT SAME THING.

i have paid 3 mechanics and all 3 fixed the problem, they thought, until the truck was cold the nxt day and it began with the same hesitating and minor missing in firing. "More of a delay or hesitating in fuel injection or? Firing? So not sure which one. But. (Not so much a "missing in firing" "I don't think?" ) Anyway.

today it is warm outside and the truck is RUNNING PERFECTLY.

when it is not, the shifts from gear to gear even seem a little less smooth, thte idle is extremely low, it has major hesitation when gas pedal is depressed (when its in gear). THEN WHEN ITS REALLY WARM IN TEMPRATURE, "IT RUNS FLAWLESS!"


ANYONE HAVE ANY IDEAS?

PLEASE HELP.
THOMAS
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Saturday, February 8th, 2014 AT 1:03 PM

5 Replies

Tiny
CARADIODOC
  • EXPERT
You can't adjust ignition timing. That is set by the crankshaft position sensor. It has some serious timing advance built in, then the Engine Computer calculates the correct amount of delay to achieve the desired actual timing advance. By turning the distributor you just changed when the injectors fire. If they fire too soon, the fuel will just puddle in the intake manifold until the intake valve opens. That gives the fuel a little time to condense back to a liquid. Liquid gasoline doesn't burn. It has to be a vapor.

For the symptoms you described, the first suspect should be the fresh air tube between the mass air flow sensor and the throttle body. Only Chrysler has never needed a mass air flow sensor to make their engines run right. For all other manufacturers that sensor has the biggest say in how much fuel goes into the engine. There can't be any cracks or other leaks in the tube. If any air sneaks into the engine that the computer doesn't know about, it wont command the fuel to go with it.

Mass air flow sensors work on temperature. They heat up a temperature-dependent resistor, (thermistor), then watch how much the air flow, by mass, cools it off. The intake air temperature sensor is used to know the starting point. Those sensors have an extremely low failure rate because they contain just one component, but if there's a less-than-perfect connection on one of the terminals, that will alter its reading. The sensor is fed from a 5.0 volt circuit, but its signal voltage must be between 0.5 and 4.5 volts, (approximately). Anything outside that range will trigger a diagnostic fault code. The problem though, with a poor connection, is the signal voltage could be within that range, but wrong. No code will be set but the Engine Computer will base its fuel metering calculations, in part, on air temperature, and on the readings from the mass air flow sensor.

If there's an aftermarket cold air system on the truck, get rid of that. The goal of the intake system is to warm the air so the fuel will vaporize better and burn more completely. The goal of cold air systems is to condense the air so more can be packed into the cylinders. You need more fuel to go with that air, otherwise you'll be running lean which causes stumbles and hesitation. Once the engine gets to around 160 to 180 degrees, the computer adds the readings from the oxygen sensors to its fuel metering calculations. At that point a lean condition will begin being detected and the computer will add more fuel to get the correct mixture. Cold air intakes are a serious waste of money. They only provide value at wide-open-throttle when you're at your limit of how much air can be squeezed into the engine. At any other speed, if you want to go faster, you just push the accelerator pedal further.
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Saturday, February 8th, 2014 AT 3:33 PM
Tiny
THOMAS GREY
  • MEMBER
Thank you for your time in trying to figure something out. Im not sure which tube your referring to; as "fresh air tube" ; and so basicly your advice is to check that tube for cracks or leaks?

It sounds like you know almost exactly what the deal is and id give my 9th born child to have you here rite now to help with this. Jk; "hope I don't ever have that many children", bad joke. That's just how desperate I am to figure this out.

Ok, so basicly it sounds like your dead on and if you can refresh me on the best step by step "things" I can do to look for and possibly solve this problem.

Thanks mechanic god

thomas
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Saturday, February 8th, 2014 AT 4:26 PM
Tiny
CARADIODOC
  • EXPERT
Yeah, I'm smart all right. I have a lot of people fooled. Actually, I don't specialize in GM products, and they confuse me because they use so many different systems and variations that I can't remember them all.

As for that tube of great importance, it connects to the throttle body. If you don't know what that is, it has the throttle cable attached to it with a quarter-round cable guide. We had a '98 lemon-law truck donated to our school that I think is the same as your truck. There is no air cleaner assembly on top of the throttle body like there was years ago when they still used carburetors. Instead, I think you'll have a black plastic rounded cover, about six or eight inches in diameter, as I recall, and a couple of inches tall, in its place. If you pull that assembly off while the engine is running, it will either start to misfire badly, or will likely stall completely. That is because air is going in that is not being measured. The Engine Computer doesn't know about that air so it doesn't provide enough fuel.

The hose I'm referring to is attached to or is part of that round cover. On our school truck it ran to the passenger side right in front of the firewall. The intake air temperature sensor is plugged into a hole in that tube very near the right hood hinge, then the tube curved forward to the air filter box on the right inner fender. Somewhere along that tube is the mass air flow sensor. The tube will be fastened to the sensor with a large hose clamp. You'll see the electronics part of the sensor on top with about six wires connected to it. That is the tube that can't have any air leaks.

If my description doesn't make sense, try to post a photo of the top of your engine, then I'll see if I can point out the tube and sensors.
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Saturday, February 8th, 2014 AT 6:31 PM
Tiny
THOMAS GREY
  • MEMBER
I posted a few pics hoping that one of the angles might be good enough. Mr. Mechanic guy; check this out. The truck had been running good all afternoon and id even went and replaced the wanter temp sensor which had been destroyed since I bought it ( unrelated) neway, I went to test to see if the truck was still running good and it was until half mile down the street the gas got STUCK literally. I had to turn the turn off the ignigition in neutral before it blew up. The I went and wiggled the throttle calbe to make sure it was not suck which it didn't seem to have been; but when I recranked it; it was back to running "not so good again", the idle was tryuing to idle way too low, and it was hesitating again when I pushed the gas pedal.
All I know is of all the people that ive talked to about this problem; you seem to be describing symptoms and issues iriely similar to what ive got going on. I really think you're on to my problem.
I posted a few pics; hoping you might be able to guide me somewhere.

Let me know.

It does seem to have something to do with "sensor" type stuff; and it being warm and cold; is MAJOR factor in whats going on.

Let me know./

forever gratefull.

Thanks.
Thomas
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Saturday, February 8th, 2014 AT 7:31 PM
Tiny
CARADIODOC
  • EXPERT
The photo didn't come through. This happens to a lot of them. If it will help, I copy my drawings into MS Paint, then save them as a. Jpg file. Those seem to work better.
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Saturday, February 8th, 2014 AT 10:07 PM

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