1990 GMC Sierra 1500 running bad

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ColoradoDale

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Bought this truck last week. Idles great, no power.
Check engine light was on, indicating bad throttle sensor and knock sensor.
Replaced o2 sensor, EGR Valve, knock sensor and throttle sensor. Has new gas tank and fuel pump. replaced fuel filter. No more check engine light, but still no power. EGR solenoid checks out. Diagnostic computer doesn't find anything out of ordinary. Checked for vacuum leaks. Has new exhaust including catalytic converter on back. New distrib. cap and plug wires.

What is causing lack of power? Runs like only using half of the cylinders.

Update: Found leaking gasket around back side of throttle body. Replaced gasket. Still running bad.
Tested fuel pressure on engine side of filter. Reads 17 PSI.
Check-Engine light comes on once in a while. Code indicates engine is running lean.
 
Last edited:
ColoradoDale said:
Bought this truck last week. Idles great, no power.
Check engine light was on, indicating bad throttle sensor and knock sensor.
Replaced o2 sensor, EGR Valve, knock sensor and throttle sensor. Has new gas tank and fuel pump. replaced fuel filter. No more check engine light, but still no power. EGR solenoid checks out. Diagnostic computer doesn't find anything out of ordinary. Checked for vacuum leaks. Has new exhaust including catalytic converter on back. New distrib. cap and plug wires.

What is causing lack of power? Runs like only using half of the cylinders.

Update: Found leaking gasket around back side of throttle body. Replaced gasket. Still running bad.
Tested fuel pressure on engine side of filter. Reads 17 PSI.
Check-Engine light comes on once in a while. Code indicates engine is running lean.
Click to expand...

Double check for anymore vacuum leaks and replace your MAP sensor if its faulty and giving a bad reading. The TBI engines run off the MAP sensor and O2 sensors to have it run right.

Also check your timing on the engine to make sure its timed right, good luck and keep us posted.

Molson02536;)
 
i bought a new set of wires once that had one bad line that made my truck run like crap...
 
-firing on half the cylinders-
will the engine rev up ?
which trans does it have?
how many miles on the truck?
have you looked behind the trans, a bad u-joint will rob a lot of power, are the rear brakes good and not dragging, is the rear axle/diff in good shape
 
Replacing the distributor solved the problem when my '90 350 had a stubborn miss on #2. I had gone through the whole routine, cap, rotor, coil, wires, spark plugs, and all of those things are a good thing to do after a long time, so no loss. My distributor was causing the code 42, weak ground for the EST. It wasn't all that expensive, and solved the miss and generally runs much smoother. It was corroded to hell. '90 350 v-8, 159,000 miles. Bought new, pretty much been outside ever since.
 
i'm get that code 42 after replacing my cap, rotor, plugs and wires on my 1991 1500...wasnt giving a CEL at all before i changed all that? it was just running like crap. now it runs much better but i'm getting the code...cant win for losing!
 
txmnjim said:
i'm get that code 42 after replacing my cap, rotor, plugs and wires on my 1991 1500...wasnt giving a CEL at all before i changed all that? it was just running like crap. now it runs much better but i'm getting the code...cant win for losing!
Click to expand...

Jim, did you by chance unhook the timing connector? That will set a code 42.
 
well my buddy said no he didnt. he did take off the distributor and says its pretty rusty...will replace when i get the money.
 
Trouble Code 42 indicates that there may be a malfunction in the Electronic Spark Timing (EST) system. During cranking, the timing is controlled by the ignition module and the ECM grounds the EST line. It expects to see no activity on this line at this time. When the ECM enters EST mode, it applies +5 volts to the BYPASS line and expects to see voltage variations on the EST line.
Reasons for the ECM to set Code 42:
? System in BYPASS mode (i.e. EST line supposedly grounded) but activity sensed on EST line
? System in EST mode (i.e. BYPASS line driven with +5 volts) but no activity seen on the EST line

Possible causes include:
1. BYPASS line is open or grounded
2. EST line is open or grounded
3. Poor connections between ignition module and ECM
4. Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise
5. Faulty or incorrect ignition module
6. Faulty ECM
 
thanks for the info Indy! of course, now the CEL went out and the brake light came on :( the trouble with this truck is just starting too. i spent a ton of money getting my 89 F150 up to snuff, but i decide i'm tired of sliding and slipping all winter so i decide to rehab this 4x4. its a good truck that i've had as a plow truck for the last 7 years and now i'm turning it into my daily driver. i imagine it will be quite a while, and quite a few dollars before its good. (in fact, i'm going to have to get a seasonal temp job just to pay for the new tires it desperately needs :)
ps sorry for hijacking this thread Dale!
 
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