Help!!

[Rustyinmonroe]

Hello all, I'm new to the forum and in my first message, I've got to ask for help My father asked me to try and find some help with a problem he's having with a truck. Here's the specifics:

It's a 1987 GMC Pickup 1/2 Ton with 5.0 liter engine. It has a Throttle Body fuel injection system.

Problem: He can't get it to pull over 10 inches of vacuum psi. He needs 17.

Here's what he's done thus far:

1.) Cleaned the throttle body.
2.) Set the timing to specifications.

Any suggestions? The new guy would really appreciate it and I could score some points with my father as well!

Rustyinmonroe

[Zkast]

Are there any running problems right now? @ 10" of vacuum it should be barely running at all. It should be very very rich. The map sensor will be almost 2.4 volts at 10" which is just over cruise vacuum. Are you sure the vacuum gauge is correct??

It's possible to have a vacuum leak at the throttle body gasket (VERY common on this setup).

Also, where is he getting this vacuum reading? from the TBI or from the vacuum tree in cylinder #8 runner.

[Hooter]

another common problem is vacum leak on the brake booster or bad guage, I have fiddled around with the vacum on my 74 forever never got it good until i used a different vacum guage and turns out it was just fine

[Rustyinmonroe]

Originally posted by Zkast
Are there any running problems right now? @ 10" of vacuum it should be barely running at all. It should be very very rich. The map sensor will be almost 2.4 volts at 10" which is just over cruise vacuum. Are you sure the vacuum gauge is correct??

It's possible to have a vacuum leak at the throttle body gasket (VERY common on this setup).

Also, where is he getting this vacuum reading? from the TBI or from the vacuum tree in cylinder #8 runner.

He told me he's used 2 different gauges and gets the same result. He also said the engine requires anywhere from 17 to 15 pounds of vacuum to work with that map sensor. He's checked the gasket and it's not leaking. He said he's getting the reading from the vacuum tree in cylinder #8 runner. Everywhere he's checked he still gets the 10 pounds. When he revs up the motor to 1500 rpm then he gets the proper vacuum and it starts running good.

Any other ideas?

Rustyinmonroe

[Rustyinmonroe]

Originally posted by Hooter
another common problem is vacum leak on the brake booster or bad guage, I have fiddled around with the vacum on my 74 forever never got it good until i used a different vacum guage and turns out it was just fine

He's checked both of those and the booster is fine. He's also used 2 different gauges and gotten the same reading. Can you think of anything else?

Rustyinmonroe

[Zkast]

If everything has been checked for leaks, your next step is to find out why vacuum is not being generated!

You say he checked the timing? Was it off and if yes, how much was it off. I"ve seen timing chains jump 1 or 2 teeth which causes late valve timing. This results in very low vacuum readings.
If the timing was more than 10 degree's off and no one has touched it in a while... this could be where your headed.
Also do a compression check, lets see what psi your running in the cylinders.

[Rustyinmonroe]

Originally posted by Zkast
If everything has been checked for leaks, your next step is to find out why vacuum is not being generated!

You say he checked the timing? Was it off and if yes, how much was it off. I"ve seen timing chains jump 1 or 2 teeth which causes late valve timing. This results in very low vacuum readings.
If the timing was more than 10 degree's off and no one has touched it in a while... this could be where your headed.
Also do a compression check, lets see what psi your running in the cylinders.

The timing was off 5 degrees when he checked it and he set it back to 6 degrees. The compression check shows it's running at 135 pounds. It doesn't vary within 2 to 3 pounds in each cylinder.

He said tonight the only thing he hasn't gotten down to and checked with the valve adjustments. Could this have something to do with it? Motor was rebuilt 3 years ago.

Rustyinmonroe

[Zkast]

If the valve adjustment is too tight, it will directly affect vacuum.


If you have ruled out every possible vacuum leak (intake gaskets, tbi gasket, booster) then your headed into mechanical problems.
Unless the valves have been adjusted recently, I doubt your problem is there, still need to check though.

When he has the vacuum gauge on a large source, does the needle bounce rapidly or is it steady at 10"?

If the needle is bouncing, you could have broken/weak valve spring(s) or even a camshaft/lifter going flat.

[Rustyinmonroe]

Originally posted by Zkast
If the valve adjustment is too tight, it will directly affect vacuum.


If you have ruled out every possible vacuum leak (intake gaskets, tbi gasket, booster) then your headed into mechanical problems.
Unless the valves have been adjusted recently, I doubt your problem is there, still need to check though.

When he has the vacuum gauge on a large source, does the needle bounce rapidly or is it steady at 10"?

If the needle is bouncing, you could have broken/weak valve spring(s) or even a camshaft/lifter going flat.

Thanks zkast, I passed this along to him. He's going to check into this, as the needle was bouncing rapidly. As soon as I hear from him, I'll holler back at you. Thanks again!

Rusty