1995 Chevy Van will not accelerate

Tiny
LEGATUS
  • MEMBER
  • 1995 CHEVROLET VAN
Engine Performance problem
1995 Chevy Van V8 Two Wheel Drive Automatic 180k miles

Have replaced TPS, Fuel filter, fuel pressure regulator. Cap wires, plugs all new. It will start and idle for a bit slightly rough, not bad though. I can feather the pedal and rev it but it will not accelerate in gear and I have to get on and off the gas to make it move at all. I can see the fuel coming out the injectors on the throttle body and it seems even and increases with the pedal but it will die if I attempt to slowly rev it up only a fast rev will work.
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Friday, July 2nd, 2010 AT 10:58 AM

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Tiny
CARADIODOC
  • EXPERT
Hi Legatus. Welcome to the forum. Your symptom sounds exactly like what happened on Chrysler products in the late '80s. It was due to a failed MAP sensor. It was developed and used by GM, and they had so much trouble with it, they sold it to Chrysler. According to a high level Carquest trainer, Chrysler is the only manufacturer that has been able to make an engine run right with just a MAP sensor. Everyone else uses a Mass Air Flow sensor in the air intake system. GM used the MAP sensor for a backup strategy in case the MAF failed. They also use it to monitor barometric pressure.

First of all, if you are watching the injector spray, you have the air tube disconnected. Any leaks in the tube or any air getting into the engine that doesn't go through the MAF sensor will not be measured so the corresponding amount of fuel will not be injected. The fresh air system must be sealed from leaks for the engine to run right.

Next, unplug the MAF sensor. If the engine runs better, clean or replace the sensor.

Caradiodoc
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Friday, July 2nd, 2010 AT 12:22 PM
Tiny
LEGATUS
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I have run it with all the sensors unplugged one at a time, the map made some difference so I changed it. It is still on its face and will not accelerate
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Friday, July 2nd, 2010 AT 1:58 PM
Tiny
CARADIODOC
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Do you have access to a gauge to measure fuel pressure? How about a plugged catalytic converter? If it's plugged, the sound from the tail pipe will be kind of a hiss instead of the normal "thump thump".

Caradiodoc
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Friday, July 2nd, 2010 AT 2:27 PM
Tiny
LEGATUS
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I could get a gauge but I would have to add a T in the line as well.
The pipe sound sounds normal to me.
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Friday, July 2nd, 2010 AT 2:34 PM
Tiny
CARADIODOC
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Yup. I have the same problem with my vehicle. No test port. Throttle body systems don't run very high pressure anyhow. Probably a better test would be to use a scanner to read the throttle position sensor voltage and especially the mass air flow reading. See if it responds to opening the throttle.

You might consider performing a fuel pump volume test. You'll have to find the specs in a service manual. If it is real low, suspect a plugged filter or the pickup screen in the tank.

Caradiodoc
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Friday, July 2nd, 2010 AT 3:55 PM
Tiny
LEGATUS
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A volume test is performed how? The stores do not even show a mass air sensor as available for that van.
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Friday, July 2nd, 2010 AT 4:07 PM
Tiny
CARADIODOC
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If there's no MAF sensor, this is one of the few GM vehicles that doesn't use one. Then the MAP sensor has the biggest say in how much fuel is commanded from the injectors. That would make sense if you were able to view the injector spray with the air inlet tube off.

To perform the volume test, disconnect the fuel supply line from the engine and run it into a container. Activate the fuel pump for the specified amount of time, then measure the volume it pumped. A typical amount might be 2 quarts in 20 seconds. On some vehicles the pump will run all the time the ignition switch is on, but usually you will have to bypass something to turn it on. The easiest way is to use a jumper wire in place of the fuel pump relay or to remove its cover and squeeze the contact. Some pumps will run if you ground the fuel pump terminal on the oil pressure sending unit but that was more common on older cars.

Usually if there is a restriction to fuel flow, it will be pretty obvious. You'll only have a little trickle of flow instead of a nice strong stream.

Caradiodoc
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Friday, July 2nd, 2010 AT 4:59 PM
Tiny
LEGATUS
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Well with whats been done it will start and idle and the ac no longer kills it. Putting it in gear with the break on will kill it but not as fast as before. I can wind the rpm's up slow or fast now it just wont perform in gear. Could this be a trans problem? Just seems to have to much gas at the throttle body to be the pump and I want that to be my last resort.
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Friday, July 2nd, 2010 AT 5:43 PM
Tiny
CARADIODOC
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Ya know, ... It almost sounds like what you're describing is what happens with a jumped timing chain. Low power, no acceleration, hard to start, etc. Do you have a 4.3L with a distributor in the back? You might want to check the ignition timing. If it is retarded, (assuming you haven't readjusted it recently), suspect a jumped timing chain.

Caradiodoc
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Friday, July 2nd, 2010 AT 6:29 PM
Tiny
LEGATUS
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I never said anything about hard to start, it runs like glass except when in gear.
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+1
Friday, July 2nd, 2010 AT 6:32 PM
Tiny
CARADIODOC
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"Hard to start" is just one possibility. Retarded cam timing will make an engine run real smooth but low power because the fuel finishes burning in the exhaust manifold instead of the cylinders.

Caradiodoc
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Friday, July 2nd, 2010 AT 7:10 PM
Tiny
LEGATUS
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Did not realize I could adjust the timing on a computer controlled electronic ignition.
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Friday, July 2nd, 2010 AT 7:14 PM
Tiny
CARADIODOC
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Only if you have a distributor with the only position sensor. I'm more familiar with Chrysler products, but other than the mass air flow sensor, GM and Chrysler are very similar in the way they do things. If you have a separate crankshaft position sensor, that will determine spark timing, not the distributor.

Caradiodoc
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Friday, July 2nd, 2010 AT 7:51 PM

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