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FIX  IT MAN
FIX IT MAN, Master Technician
Category: GMC
Satisfied Customers: 17198
Experience:  Master Automotive Technician
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GMC Sierra: 1990 GMC Sierra*****5.7 TBIRuns great

Customer Question

1990 GMC Sierra*****5.7 TBI
Runs great for until warmed up...10 or 15 minutes. Then it will stall/bog down...have to feather the throttle to keep it going and even then 30 or 35 is tops.
It will still idle fine at that point...it also starts up easily when it is warmed up.
My fuel pump shows 13 psi...backpressure is about a pound or pound and a half.
vacuum is 17 or so...
have changed coil, plugs, temp sensor. The rotor and cap look new. I have put some seafoam and chemtool in it.
it was throwing EGR code but the hook up was all goofy...fixed that and the code went away.
other code is MAP sensor.... but the service engine light is not on, no service lights are on.
i am not a mechanic, but have worked on/at cars since my first one in '75.
Submitted: 1 year ago.
Category: GMC
Expert:  FIX IT MAN replied 1 year ago.
Hello, my name is***** sounds like the problem might be the MAP sensor. This is the most common sensor to cause this. Tap on it with a screwdriver with the engine running and see if the idle changes. If it does then replace it. If not then remove the ignition module from the distributor and take it to a parts store to have it tested. It may be failing when it gets hot. Let me know if you need more help.
Customer: replied 1 year ago.
is the ignition module the coil pickup? Would it be the MAP sensor even if it was idling very well? It idles good even after it is warmed up...
Expert:  FIX IT MAN replied 1 year ago.
Yes, the MAP senor affects fuel metering. As for the module, no it is a small plastic module inside of the distributor mounted to the base of it.
Customer: replied 1 year ago.
I will tap the sensor and get back to you. When I tap it what kind of change in idle should I look for? Will it begin to idle poorly, or faster, or ?
Expert:  FIX IT MAN replied 1 year ago.
Any change at all and it is bad.
Customer: replied 1 year ago.
I didn't notice a change when I tapped on it. Checked the voyage on it and it was 5.7. When I used s vacuum hose to suck on it the voltage did not change...think it should decrease. Also the code reader said the voltage was too high on the map sensor. What do you think? Is there any thing else I can check it on?
Expert:  FIX IT MAN replied 1 year ago.
It should change. You may want to change it. I would still have the module tester first before spending any money.
Customer: replied 1 year ago.
Module tested OK. Ran it 3 times..... Changed out the MAP sensor like you suggested. Still does the same thing. Once it gets warm it bogs down under any load and loses power. It will still idle just fine and will start right back up if it dies or if I turn it off.
Expert:  FIX IT MAN replied 1 year ago.
Do you have a voltage meter to test the TPS?
Customer: replied 1 year ago.
TPS checks out to be good....
Expert:  FIX IT MAN replied 1 year ago.
Do you have a vacuum gauge?
Customer: replied 1 year ago.
Vacuum is 17 to 20
Customer: replied 1 year ago.
Are you still there? My truck is still not running right. the vacuum is 17 to 20. I cleaned up and rebuilt the throtle body using a kit. The truck still starts fine, idles fine, and runs fine for 15 or 20 minutes and then starts to bog down and backfire. have to really tickle the gas pedal to get it to move....and uphill is just about impossible. Before it gets warm it runs great...accelerating to 70 quickly and easily. what is the next thing to check?
Expert:  FIX IT MAN replied 1 year ago.
I am assuming that is vacuum at idle, correct? If so hold the engine at about 2500 rpm and tell me what it is once it first reaches that and what it is after holding it there for about 60 seconds.
Customer: replied 1 year ago.
At idle.....20. At approx 2500 rpm.....just over 20. Stays there after 60 seconds.
Expert:  FIX IT MAN replied 1 year ago.
Did the vacuum first spike when you hit that higher rpm?
Customer: replied 1 year ago.
Yes
Customer: replied 1 year ago.
Now that I'm thinking about it I'm not sure if it first spiked or dropped. Will check it again and let you know...
Customer: replied 1 year ago.
When I first start going up to 2500 rpm the vacuum drops briefly and is then steady at 20 pounds. When I let off the throttle the vacuum spikes briefly and returns to 20
Customer: replied 1 year ago.
Hello?
Expert:  FIX IT MAN replied 1 year ago.
Sorry, I was not online earlier. I am not online very often during the day. If you unplug the hose from the EGR valve and plug it off how does the truck run?
Customer: replied 1 year ago.
The same
Customer: replied 1 year ago.
It runs the same....I really need more help on this, should I call Just answer and get another mechanic?
Expert:  FIX IT MAN replied 1 year ago.
Does you truck still have the factory catalytic converter on the exhaust?
Customer: replied 1 year ago.
Yes it does...had it checked and there is not a problem with backpressure......I think it was about a pound and a half or so....