1995 Chevy Van will not accelerate

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

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

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

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

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

"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

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