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John
John, Master A.S.E Automotive technician
Category: Chevy
Satisfied Customers: 9176
Experience:  A.S.E MasterTechnician A1-A8 w/L1,38 years experience General Motors Master Technician
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1990 C1500 TBI 5.7L Idle is surging..MAP Sensor..goes away..Autozone

Customer Question

1990 C1500 TBI 5.7L Idle is surging when warm. Unplug the MAP Sensor and the problem goes away, I replaced the MAP Sensor with one from Autozone, and that didn't fix the problem. Thoughts?

Thanks Bob
Submitted: 4 years ago.
Category: Chevy
Expert:  John replied 4 years ago.

HelloCustomer

Your problem can be several different things.I always start with a scan tool to check for failure coses & then view the data with engine off/key on to look at the TPS (should be around .50 v & not moving with throttle closed),engine coolant temperature,& MAP voltage.Thre voltage you see on the scan tool should be the same as you see when disconnecting the MAP & testing with a digital volt meter.

Check the EGR for being partially open (this may require removal to look at pintle)

With engine running look at fuel integrator & block learn (fuel trim) they should be close to 128.If they are higher the computer is adding fuel.When the engine is warmed up the 0/2 should be bouncing back & forth over .500 mv.

Customer: replied 4 years ago.

Hi John,

 

My BLM is 135, sorry I forgot to mention i replaced the 02 sensor. Bob

 

Expert:  John replied 4 years ago.
OK, you have a slightly lean condition,from here you have to check & figure out why.Do the other checks I advised & use a can of carb spray (in short bursts) to check for vacuum leaks around the TBI.
Customer: replied 4 years ago.

Hi John,

 

Thanks John,

 

I will do the checks and let you know, thanks for your help.

Expert:  John replied 4 years ago.
OK,I hope you have a good day.
Customer: replied 4 years ago.

OK, John, I replaced the EGR valve with a know good spare, also checked for leaks around the TBI. Still same issue. NOTE-when I unplug the MAP Sensor the problem goes away, but MAP Sensor has been replaced. I put a scan tool on the obd1 and checked the voltage on the MAP sensor. when the truck is running it sits around 0.90 volts, when the truck off of, key on, the voltage is 4.70. Reading the tech manual it says engine off, key on, voltage should read .4 volts, could I have a bad ECM??

 

Bob

 

Expert:  John replied 4 years ago.
You could have a bad ECM,but the reading with key on.engine off is correct.This is a 5 volt circuit.The sensor voltage with key on.engine should be around 4.50 to 5.00 V.Check the 3 circuits from the MAP to the ECM.Check the ignition timing,making sure you have advance at idle.
Customer: replied 4 years ago.

Thanks John,

 

I will look at this later this afternoon, will let you know the outcome of the readings, etc.

 

bob

 

Expert:  John replied 4 years ago.
OK
Customer: replied 4 years ago.

John, you online?

 

bob

 

Customer: replied 4 years ago.

Hi John,

 

Timing checks good at 0 degrees. I tested the voltage at the sensor connector KEY ON, ENGINE OFF. Circuit 416, GRY Wire (had meter set at 20 DCVolts) meter showed 7.00 volts. NOTE-SCAN TOOL shows 4.70 volts. CKT432 LT GRN Wire 00.6 volts, CKT455 Puple wire showed 00.7 volts.

 

Thoughts??

 

Thanks Bob

Expert:  John replied 4 years ago.

Hello Bob,

What is timing after the bypass circuit is reconnected ?

The only voltage reading that is important is the 4.7 V.The other circuits are MAP sensor signal to ECM ,& sensor ground.The sensor ground should test .002 OHMS or lower.

 

Customer: replied 4 years ago.

Hi John,

 

When reconnecting the bypass connector, the timing was not even close to the timing the timing mark.

 

Bob

 

Expert:  John replied 4 years ago.
It should not be ,you should have about 28-32 degrees advance.
Customer: replied 4 years ago.

Ok, so where does this leave me?

 

 

Expert:  John replied 4 years ago.

Have you tested the MAP sensor wiring from engine to ECM with an ohm meter ?

Look very closely at the terminal ,they must make good connection.
Customer: replied 4 years ago.
will check that now, be right back
Expert:  John replied 4 years ago.
OK
Customer: replied 4 years ago.

OK, 170 oms on Pupple wire and 302 ohms on the green wire.

 

Bob

Expert:  John replied 4 years ago.
If this reading is with the sensor & ECM disconnected you have a high resistance circuit,If not go back disconnect the MAP & ECM so the circuit can be tested.
Customer: replied 4 years ago.

I might of done this wrong,

 

i only put the ohm at the connector of the sensor and checked there with key on/eng off.

 

Bob

 

Customer: replied 4 years ago.

also with the key off it reads, GRY - 006 ohms / Green - 052 ohms / Pupple- 0 ohms

 

Bob

 

Expert:  John replied 4 years ago.
What you need to do is test the circuits from one end to the other MAP sensor end & ECM end,with the components disconnected.
Customer: replied 4 years ago.

Ok, i will head to the auto parts store and get long enough wire leads for the ohm meter,

Thanks John, i will send you a reply when I finish that.

 

I suspose we are seeing if there is resistance is in the wiring or it's a ECM issue, RIGHT?

 

Bob

 

Expert:  John replied 4 years ago.
Yes.
Customer: replied 4 years ago.
Can't find anyone around here that has the extra long wiring for a meter, still looking
Expert:  John replied 4 years ago.
I have opted out & another expert willassist you with this problem.
Customer: replied 4 years ago.

Hi John,

 

I took my Truck to GM to put on their diag tool. They couldn't figure out what the issue was either. They did tell me that my distributor was cracked, but didn't think that was the issue anyway, long story short, I replaced the distributor with a new one which included a new pickup coil, ignition module, NO HELP, I also replaced the computer with no help. Get this - I replaced the spark plug wires and the problem cleared. I did find a short in one of the wires. (never did find the crack in the distributor)

 

Bob

 

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