Mechanics

TRUCK CUTS OFF IN GEAR

1990 GMC C1500 • 225,000 miles

I have a 90 gmc 1500 truck with 5.7 liter 350. It started running rough and trying to cut off at stop lights on my way to work. Checked the codes and had a 32 egr valve and a 44 lean o2 sensor. Looked at the plugges and they are white on the ends. I did some checking and everything seemed to point to low fuel pressure so I changed the fuel pump and filter but that didnt fix it. I found a vacume line broke and fixed it but still have the problem with it cutting off. I swapped out the egr valve and the temp sensor with ones on my 95 suburban with no luck. The distributer, rotor button and cap all look good. Right now the truck runs great in park and neutral but when you put it in drive it will idle until I hit the gas then its acts like it ran out of fuel. It doesnt choke down like the converter is locked or anything just acts like it runs out of fuel. It also doesnt act like its as bad in reverse as drive. In park and neutral it will rev up and idle like nothing ever happened. Thanks for any help you can give.
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Jeffskw
April 13, 2013.




Check under the tbi unit for a vacuum leak, just spray some choke cleaner around it and if rpm changes you found it. They tend to blow the gasket beneath the unit. Also make sure the hose leading to map sensor on firewall isn't soft as that screws up the map sensor
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Tiny
Hmac300
Apr 14, 2013.
Ok I sprayed everywhere on the top of the engine and around the throttle body and no change in the rpm. Sprayed everything that had a vacume line with no change. No hoses messed up either. The truck cranks up soon as you turn the key and idles great. I can rev it to 2000 or 3000 rpm for as long as I want and runs great. I put it in drive with no change but when I hit the gas it started to go and shut off as if I turned off the ignition.

Tiny
Jeffskw
Apr 16, 2013.
Check fuel pressure it should be 9-11si then check your wires to injectors they may be loose and losing contact. Do you still have the same code. One other thing to check is the wires from the pickup coil to ecm in cap, they can get bad as they are very small when you try to go it knocks out ecm check pick up coil resistance with an ohm meter and thepickup disconnected from the ecm wiggle the wires and if resistance changes then you may need a new pick or ecm. Also check to see if the O2 sensor wire has hit the manifold and is grounding out as that can cause one of your codes. Check eger passages for being blocked. Check idle air controlmotor A@B it shold be 40-80 ohms and C@D same reading then check B_C and A-D that shold be infinite if not replace the iac.I really have no other suggestions for you that I can think of. Oh one other thing check those to map sensor if soft replace it as that may be the problem as well it's on the firewall

Tiny
Hmac300
Apr 16, 2013.

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