90 GMC C2500 Vandura 350 Severe Loping

Rex7 · 07-04-2008, 03:10 AM

Hello, great forums! I have a 1990 GMC C2500 Vandura 350 which is loping severely (very high idle to almost dying) while in neutral as well as in drive. Sometimes it dies when the idle in on the low side of the cycle and while in drive, slowing to stop. Every now and then, it will idle steady at a very high idle, but not for long. It has lost power. It has gradually been getting worse on all of the above. Check engine light comes on after a while of running, giving the code 44: Oxygen sensor or circuit; lean condition detected.

I have replaced the following:
oxygen sensor(drivers side)
idle air valve
spark plugs and wires
dist. cap +rotor bug
air filter
fuel filter

I was thinking fuel pump next but don't they just go at once?

Any suggestions as to where to look next would be appreciated, as I am trying to avoid putting it in the shop. Thanks!!

PAPA BEAR · 07-04-2008, 03:18 AM

check for large vacume leak on or around intake good luck

Rex7 · 07-04-2008, 06:51 AM

I could not locate an air leak. I checked all the vacuum hoses. I don't know how to tell if there was an air leak in the tb itself other than a crack, but there was none visible.

I also cleaned my ground strap this morning.
Tested the tps and it was steady up through the range of my pedal.
I wasn't able to get a good reading on my coolant temp sensor (due to position of it) but I don't think that's my problem because it is the entire range, not just when it's cold or warm.
How do you est an EGR?

Thanks for the reply!

Rex7 · 07-04-2008, 06:31 PM

Today I verified the folowing:
not a vacuum problem
have good spark
have a slightly clogged catalytic converter which I have cleared by taking the cat off.
Replaced EGR valve.

z07 · 07-04-2008, 07:35 PM

Check for spark at each plug. Check timing.

The clogged cat makes me think either an excessive amount of unburn fuel or engine oil is getting out the exhaust.

It's time for a cylinder compression test then.

There are shops with an electronic device that can accurately measure the compression for each cylinder by monitoring the starter current while the starter spins the motor. I think for a van this would be the ideal solution because of what a ball-ache getting to the sparkplugs is.

But if you want to try it the traditional way:

There are two fundamental types of compression test. Compression and leak down. The leak down tester needs a supply of shop air. If you remove the sparkplugs and install this tool one at a time you will find if any cylinders are very different from the others. If you find a cylinder that is low, pour 2 tbl spoons of engine oil into the sparkplug hole and test again. If compression goes up then the problem is rings. If the compression stays the same then the problem is in the head or the head gasket.

The tools for testing compression are about $40 each. Your local parts store may have a tool lending program too.

Rex7 · 07-06-2008, 05:44 AM

Thanks for the reply!

I broke down and put it in the shop yesterday. After about 4 hours of testing timing and other stuff, they tested the fuel pressure and they could not get it to go above 8 psi. Specs call for 9-13 psi, so I had them replace it. It runs better than it did when I bought it now

I had thought that fuel pumps were a working or dead part....guess not heh.

Thanks for all the help here...there were other problems with the van that I solved myself thanks to this forum!

07-04-2008, 03:10 AM	#1 (permalink)
Rex7 Junior Member Join Date: Jul 2008 Posts: 4	90 GMC C2500 Vandura 350 Severe Loping Hello, great forums! I have a 1990 GMC C2500 Vandura 350 which is loping severely (very high idle to almost dying) while in neutral as well as in drive. Sometimes it dies when the idle in on the low side of the cycle and while in drive, slowing to stop. Every now and then, it will idle steady at a very high idle, but not for long. It has lost power. It has gradually been getting worse on all of the above. Check engine light comes on after a while of running, giving the code 44: Oxygen sensor or circuit; lean condition detected. I have replaced the following: oxygen sensor(drivers side) idle air valve spark plugs and wires dist. cap +rotor bug air filter fuel filter I was thinking fuel pump next but don't they just go at once? Any suggestions as to where to look next would be appreciated, as I am trying to avoid putting it in the shop. Thanks!!

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07-04-2008, 03:18 AM	#2 (permalink)
PAPA BEAR Senior Member Join Date: Mar 2008 Location: North Carolina Posts: 86	check for large vacume leak on or around intake good luck

07-04-2008, 06:51 AM	#3 (permalink)
Rex7 Junior Member Join Date: Jul 2008 Posts: 4	I could not locate an air leak. I checked all the vacuum hoses. I don't know how to tell if there was an air leak in the tb itself other than a crack, but there was none visible. I also cleaned my ground strap this morning. Tested the tps and it was steady up through the range of my pedal. I wasn't able to get a good reading on my coolant temp sensor (due to position of it) but I don't think that's my problem because it is the entire range, not just when it's cold or warm. How do you est an EGR? Thanks for the reply!

07-04-2008, 06:31 PM	#4 (permalink)
Rex7 Junior Member Join Date: Jul 2008 Posts: 4	Today I verified the folowing: not a vacuum problem have good spark have a slightly clogged catalytic converter which I have cleared by taking the cat off. Replaced EGR valve.

07-04-2008, 07:35 PM	#5 (permalink)
z07 Senior Member Join Date: Feb 2008 Posts: 176	Check for spark at each plug. Check timing. The clogged cat makes me think either an excessive amount of unburn fuel or engine oil is getting out the exhaust. It's time for a cylinder compression test then. There are shops with an electronic device that can accurately measure the compression for each cylinder by monitoring the starter current while the starter spins the motor. I think for a van this would be the ideal solution because of what a ball-ache getting to the sparkplugs is. But if you want to try it the traditional way: There are two fundamental types of compression test. Compression and leak down. The leak down tester needs a supply of shop air. If you remove the sparkplugs and install this tool one at a time you will find if any cylinders are very different from the others. If you find a cylinder that is low, pour 2 tbl spoons of engine oil into the sparkplug hole and test again. If compression goes up then the problem is rings. If the compression stays the same then the problem is in the head or the head gasket. The tools for testing compression are about $40 each. Your local parts store may have a tool lending program too.

07-06-2008, 05:44 AM	#6 (permalink)
Rex7 Junior Member Join Date: Jul 2008 Posts: 4	Thanks for the reply! I broke down and put it in the shop yesterday. After about 4 hours of testing timing and other stuff, they tested the fuel pressure and they could not get it to go above 8 psi. Specs call for 9-13 psi, so I had them replace it. It runs better than it did when I bought it now I had thought that fuel pumps were a working or dead part....guess not heh. Thanks for all the help here...there were other problems with the van that I solved myself thanks to this forum!