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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: robosmurf on January 26, 2018, 07:22:54 am
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I'm having issues with my truck going into limp mode.
I warmed her up last weekend for about 15 min and then headed down the highway at 60mph for about 10 miles. SES light came on and then at the next stop sign it was stumbling hard. I had to keep the gas pressed down and the other foot on the brake to keep it running.
Made it home and pulled codes 32 and 33, EGR and MAP. Would the two together indicated a vacuum leak instead of a bad sensor? I just went through the TBI on this one, I had to rebuild the rebuild that a previous owner put on with a new TPS and new fuel regulator spring.
Can any of the parts stores test MAP sensors?
'87 454 w/TBI
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limp mode imo would be a speed limiting feature like nothing above 25. what youre describing sounds like a vary large vacuum leak and the engine just wants to die. egr is only active under certain conditions like cruising and throttle usage less than 30% (cant remember the exact numbers but its close to that) something that can happen is carbon can get stuck in the egr causing it to be stuck open and that will cause what your describing. sometimes you can remove and clean the egr, also check for other obvious vacuum leaks
also test the map sensor
http://forum.73-87chevytrucks.com/smforum/index.php?topic=29996.0
M.A.P.
(Manifold Absolute Pressure)
Symptoms of a Defective MAP
The 'Check Engine' Light is On
Surging
Rough idle
A rich fuel condition, which may cause spark plug fouling
Detonation due to too much spark advance and a lean fuel ratio
Loss of power and/or fuel economy due to retarded timing and an excessively rich fuel ratio
A vacuum leak will reduce intake vacuum and cause the MAP sensor to indicate a higher than normal load on the engine. The computer will try to compensate by richening the fuel mixture and retarding timing -- which hurts fuel economy, performance and emissions.
Testing
Check the reference voltage supply at pin C of the MAP sensor (gray wire typically).
Measure voltage between pin A (ground) and Pin C (+5 V ref) with a DVM. It should be right around 5 Vdc.
With Key on engine off the MAP voltage (Pin B, light green wire typically) should read around 4.8 to 4.9 volts. If it reads the same as pin C the sensor is probably is history.
Also check ground reference pin A. With everything off, measure resistance between Pin A and electrical ground (stud next thermostat). It should be fractional ohms
Manifold Vacuum
kPA 0 10 20 30 40 50 60 70 80 90 100
Hg 0 2.9 5.9 8.9 11.8 14.8 17.7 20.7 23.7 26.7 29.6
Volts 4.9 4.4 3.8 3.3 2.7 2.2 1.7 1.1 0.6 0.3 0.3
W.O.T. Idle
Manifold Pressure
kPa 100 90 80 70 60 50 40 30 20 10 0
Hg 29.6 26.6 23.7 20.7 17.7 14.8 11.8 8.9 5.9 2.9 0
Volts 4.9 4.4 3.8 3.3 2.7 2.2 1.7 1.1 0.6 0.3 0.3
W.O.T. Idle
(http://forum.73-87chevytrucks.com/smforum/index.php?action=dlattach;topic=29996.0;attach=36774;image)
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I dunno how you guys put up with people like me! You were right about the EGR being stuck WIDE open. Do I just clean it with carb cleaner? Should I oil it at all?
Thanks as always
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Is it stuck because the valve is sticking or is the solenoid sticking open? Pull the vacuum hose off of the valve and see if vacuum is present when it starts running poorly. If it is, replace the solenoid.
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It was stuck open with the vacuum disconnected. Once I got out freed up with a little carb cleaner it moved pretty well. I could compress out by hand and if I plugged the vacuum port with my finger it would stay open.
The solenoid was replaced pretty recently by the looks of it but not by me.
Sent from my SM-G920V using Tapatalk
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if i was in your shoes (and me being frugal as some would say) i would clean up your old egr with carb cleaner even go as far as removing it and cleaning underside (dont oil) and see how long it will last. if it gives you more problems then replace