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Bad MAP sensor?

jamber2541

jamber2541

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When in drive at a stop the map sensor jumps and reads anywhere between 2.75 and 3.55 on the autoxray, whenever it does this the engine stumbles. Once you step on the gas it all is well again. Could this be a map sensor problem or something else like a vacuum leak? By the way the reading of the map sensor in park is 1.23. Any ideas.
 
I vote for a vacuum leak. Check where the hose goes into the MAP. It is usally cracked there. Are you pulling any codes? Check engine light on? There is also a voltage test you can run.

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.

Another functional check – with engine idling MAP output voltage (depending on idle vacuum) should be in the neighborhood of 1.2 to 2.0 volts. If it is stuck at 5 volts check grounds and most likely MAP sensor is history.

Map readings will de different depending on your application ie: cam selection, intake, heads, ect. but should be somewhat consistant (not bouncing all around at a given load point) An abnormal reading could indicate a vacuum leak. In my opinion, the best way to find one is to do a visual first. If nothing is found but you still still suspect a vacuum leak, spray a little carb cleaner at each connection a listen for a change in idle speed. Also around the base of the TBI,
 
As said above check for vacuum leaks. Espically the TB base gasket.
If no vac leaks. Check/clean the EGR valve. It could be dirty and sticking open a bit.
 
no codes and no vacuum leaks, the EGR valve was getting stuck so I replaced it, it ran good for about 10 minutes and then it started again, the new EGR valve is also getting stuck. As soon as I unplug it everything is back to normal. Any more ideas?
 
Is the solenoid shot? Thinking maybe it's not getting stuck, instead vacuum is getting through the solenoid when it shouldn't?
 
Yeah prolly is the solenoid. If your EGR is getting vaccum at idle there is a problem with the solenoid. It is stuck open and letting vac to the egr when the egr should be closed and getting no vaccume
 
changed the solenoid and no change, the map sensor reading still jumps to around 3.20 when I come to a stop and the engine wants to die.
 
Only under braking? Bad brake booster line, check valve, or booster? Replicated when you are just sitting and hit the brake?
 
Sorry, should have given a little more info, I has done this while in park or in drive at a red light, with the brake pedal pressed down and without the brake pedal being pressed (when in park)
 
I should have posted this eariler but it slipped my mind.
Try unplugging the wires to the MAP sensor. If that cures the problem, MAP is most likely bad. Unplugging the MAP puts the computer in bypass, or limp home. It should run better if map is bad.
 
tried that and it did not get better, the only time it got better was when I pluged up the EGR line.
 
Unplug the electrical connector from the solenoid. Do you still have the problem?

Prior to this happening did anyone remove the vac lines from the solenoid? IIRC one to the base of the TBI and one to the erg. Did these get swapped on the solenoid?

Any codes?
 
Or a line hooked up to the vent "fitting" on the solenoid. Common mistake if the filter is missing.
 
no codes and the connections are correct, did not unplug the solenoid but I did unplug the vacuum line to the egr and covered the opening, this did smooth out the engine again once the EGR valve closed.
 
So did you check to see if the EGR solenoid wiring is good? Unplug the solenoid, see if problem is gone. If the problem doesn't go away, something is wrong with the vacuum line hookup, or the solenoid is faulty, period, since removing vacuum solves the problem.

Not looking at a wiring chart, I assume the EGR solenoid gets a constant 12V with the engine on, and the ECM commands EGR ground when the right conditions are met. No matter how it's wired up, either there will be 12V on one wire, and no path to ground (engine running, idling in park) or no voltage and one ground. I almost guarantee its 12V constant and no ground though. At least that's how it SHOULD be at idle, since EGR only operates at a steady state cruise condition.
 
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It must be something with the solenoid. The EGR should not be open at all when the engine is at idle speed. There should be no vaccum going to EGR from the solenoid at idle.
As dorian said above.
Are you sure the line going to the EGR is not hooked up to the vent port on the solenoid?
If the solenoid is good, that is the only thing I can think of that will give Vac at idle.
 
When you took off the Vac line on the EGR and the truck running, did you have any vacuum on the line? If you did the solenoid or wiring is bad.

Dorian was right 12v Constant and the ECM regulates the ground.

Please post the fix so future searchers can be helped!!
 
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