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Service engine light 98 GMC 1500 4X4

Tiny
DFDRODNEY
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  • GMC C1500
98000 miles. Code read cyl. 5 missfire, fulll tune up done, code stilll occassionally flashes usually goes back out after a few miles. Seems to run ok, sometimes 300 or more miles before reappearing. Also oil pressure gauge started today usually runs 30-50 willl occassionally fall immediately to 0 then after 30 sec or shoot back up replaced with aftermarket sender did fine for about 20 minutes then same thing, I don't hear any noise from engine when pressure drops is alll this related? You advice is greatly appreciated1 thanks guys sorry 5.7 vortec as far as last answer fuel pump? I am lost
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Wednesday, January 3rd, 2007 AT 8:27 PM

9 Replies

Tiny
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The connectors for those fuel pumps I believe need to be replaced when you do the pump, need to get from the stealer.

What size motor?
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Wednesday, January 3rd, 2007 AT 8:49 PM
Tiny
DFDRODNEY
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5.7 vortec I didn't replace the fuel pump? Should I? For erratic oil pressure reading and service engine light?
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Wednesday, January 3rd, 2007 AT 9:08 PM
Tiny
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:oops: Sorry it's getting late, I thought you had replaced the pump. Yikes!

Should hook up a gauge to the motor for the oil pressure and monitor it when the symptoms happen. The gauge may not be accurate.

Full tune up meaning CAP, rotors, wires, plugs, fuel filter? More?
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Wednesday, January 3rd, 2007 AT 9:23 PM
Tiny
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Yes full tune including a coil, with the 3 wire sending unit will that not shut engine down if I put mechanical gauge in place
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Wednesday, January 3rd, 2007 AT 9:27 PM
Tiny
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The misfire is always on # 5. You say that the code flashes every 300 miles or so. Do you mean the CEL flashes or you check the code when the light comes back on, but the light isn't actually flashing?

Have the intake valves been cleaned with a good cleaner?

I don't know that the problems are related. It would be good to verify the oil pressure drop if it is real or note.
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Wednesday, January 3rd, 2007 AT 9:37 PM
Tiny
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The misfire is always on # 5. You say that the code flashes every 300 miles or so. Do you mean the CEL flashes or you check the code when the light comes back on, but the light isn't actually flashing?

Have the intake valves been cleaned with a good cleaner?

What did the #5 plug look like when you changed it?

I don't know that the problems are related. It would be good to verify the oil pressure drop if it is real or note.
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Wednesday, January 3rd, 2007 AT 9:37 PM
Tiny
DFDRODNEY
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It cel comes on, I have had it read 3 times it will go back off after a while if I don't reset it. Always # 5 what would be a good cleaner for valves, I have always taken meticulious carewhen the gauges drops to zero I hear no noise was going to try oem oil pressure sensor tomorrow? When I t first came on I tuned it and plug 5 was fouled more than others slightly the cap and rotor were corroded on 5 also.
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Wednesday, January 3rd, 2007 AT 9:49 PM
Tiny
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For what ever reason, those engine go through cap and rotors, I don't know the actual reason why.

I like to use castle hydroblast. You spray it into a vacuum line such as the brake booster line if nothing else is accessible. Full spray for 3 minutes, stall the engine with it or have somone kill it while your spraying. Let it set for a few hours then take for a short drive in low gear, revving the rpms to help break it up. First use Castle cleansrite to clean the throttle bore. Try to get the throttle plates scrubbed good with a brush, while your spraying. Use a rag to wipe the carbon out between brushngs. Seafoam is another good product, but smokes a lot.

Changing the sender again, may still not tell you the real pressure, saying the gauge is bad or there is a ground problem. If you're going to try a different one, be sure the connector is clean. You may be able to test the gauge with a test light, but being a service writer that like clean hands :wink:, I can't help you on that.
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Thursday, January 4th, 2007 AT 5:31 AM
Tiny
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These also have poppit valve troubles. Sometimes a professional fuel injection cleaning helps.
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Thursday, January 4th, 2007 AT 5:23 PM

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