I have a 95 GMC g2500 Vandura I have been working on. When it was brought to me it did not have any power and would bog down when accel. I gutted out the converter that was clogged and it still bogged down. come to find out the van had sat for nine years and the gas tank was slam full of rust so the fuel pump, filter, and gas tank was replaced along with a rebuild of the throttle body. The van ran amazing after that but now is running extremely rich and has a code 34 (map sensor) and I cannot figure out what is causing this to run so rich please help… it has a 5.7
I would think the “Vacuum Leak” or failed sensor.
Code 34 – Manifold Absolute Pressure (MAP) Sensor Circuit
(Signal Voltage Low – High Vacuum)
Circuit Description
The manifold absolute pressure sensor (MAP) responds to changes in manifold pressure (vacuum). The ECM receives this information as a signal voltage that will vary from about 1-1.5 volts at idle to 4-4.5 volts at wide open throttle.
A scantool displays manifold pressure in volts. Low pressure (high vacuum) reads a low voltage while a high pressure (low vacuum) reads a high voltage.
If the MAP sensor fails the ECM will substitute a fixed MAP value and use the throttle position sensor (TPS) to control fuel delivery.
Problem Description:
Code 34 will set when:
Signal voltage is too low (kPa less than 13) and ignition is turned “ON”.
OR
Engine running greater than 1200 rpm and throttle open greater than 25%
thank you for your response but I tested for a vacuum leak with a smoke machine there were no leaks so I went and bought a new map sensor and that also did not fix the problem. it runs so rich it keeps fouling out plugs I was wondering if it might be possible that a valve or something might be causing the problem ive done ever bit of testing except for a compression test I really would appreciate any advice you might have on these thank you so much
ive also always heard to hold a dollar bill up to the tailpipe and if it tried to suck it back in for a split second you have a burnt valve so I tried it and sure enough it tried to suck it back in… is this true or have u heard anything close to this before
If you suspect a bad valve, a compression test would be the way to go.
If you still have the Code 34, I would continue to concentrate on that until it is fixed.
Here is an good source on how to perform Scan Diagnostics. You follow the test and depending upon your answer(YES or NO) it will guide you through to the next thing to check. It is for a Pontiac, but the testing for the Map Sensor still applies.
Good reference: http://www.fieros.de/en/v6help/code34.html
I checked the compression today and it was within spec but back to the map sensor deal like I said I bought a new one and it didn’t fix the problem but ive looked around on the internet and it says the bottom throttle body gasket is bad about leaking and I did not replace this when I rebuilt the throttle body I only removed the top half from the intake could this be causing that map sensor code because ii have looked and looked and can not find any kind of vacuum leaks anywhere thanks for helping me with this
ok buddy I found a vacuum line on the backside of the throttle body that was bad I also replaced the bottom throttle body gasket and all the trouble codes disappeared no more o2 running rich or map sensor however in the process it backfired and blew the muffler completely in half and it will not idle nor run worth a shit at this point and still continues to smoke like its running rich. I guess my question is will the muffler affect the back pressure or anything back enough to keep it from ideling or what obviously im still missing something since the codes went away but still continues to do the exact same thing as before
the last thing im going to bother you with is how can I set the base timing on this pos I can no find any markings are all on the timing cover or anywhere to set the timing
The back fire that blew the muffler and the running rich issue has put too much fuel through the engine. This will load up the exhaust and load up the oil in the engine.
Drain the oil and replace the oil filter. This will help. Then you will need to let it run for a good while to help the exhaust get hot enough to burn off anything still left in it. Now if the muffler blew apart, it probably damaged the catalytic converter as well. A back pressure gauge can be inserted into the upstream O2Sensor port and tested. should see about 1.5 psi at idle and no more than 3 psi at or above 1500 rpm. If you do, you have a clog, might be the converter or behind it where the muffler blew apart.
After you run the engine for a couple hundred miles, change the oil again. this should take care of the blue smoke.
Distributor Ignition (DI) Systems
1995 MODELS
NOTE: Refer to the underhood label for the proper timing setting.
1.Engage the parking brake, block the wheels and set the transmission in P.
2.Disconnect the Ignition Control (IC) system by disengaging the “set timing connector”. This is a single wire sealed connector that has a tan with black stripe lead. This wire comes out of the wiring harness below the heater case.
3.With the ignition switch OFF, connect the timing light pickup lead to the No. 1 spark plug wire.
4.Start the engine and point the timing light at the timing mark on the balancer or pulley and check the timing.
5.If the timing is not within specifications, refer to the underhood emission sticker and loosen the distributor hold-down bolt. Slowly rotate the distributor until the proper timing setting is achieved.
6.Tighten the hold-down bolt and recheck the timing.
7.Turn the ignition OFF, remove the timing light and engage the “set timing” connector.
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