Code 33
 

1998 502 MPI, code tool flashes code 33. Engine stalls on occasion. It there a simple way to verify that there is something wrong with the MAP sensor?

Re: Code 33
 

here you go hope this helps

Re: Code 33
 

Thanks GOODT. the procdure points to the sensor being faulty. Can that be verified without a scan tool. How do I measure output voltage with the MAP sensor in operation. Do I pierce the insulation on the output wire with the DVOM ?

Re: Code 33
 

The ignition on/ engine off voltage is 4.9 and the idle voltage is 2.70. Sounds very high to me. Could that be the problem?

Re: Code 33
 

Cleared the codes and put in another MAP sensor. Engine still stalls after some 5 minutes of driving. Checked for codes, it flashes 33. Checked voltage, ignition on/ engine off 5.12 Volts, engine idle 2.84 Volts. Followed the trouble shooting procedure measuring voltage A to C (5.03 V), B to C (4.75 V)and with ignition off probing B with a test light (No light). The procedure says; replace MAP sensor. ?!

I am at loss. Could it be a vacuum leak? Where to put a vacuum guage on a 502 MPI?

Re: Code 33
 

I would put it on a spx or diacom laptop and check whats going on and what kind of readings it getting in the long run if you have to pay someone to check it out its still cheaper then the time and parts youll be putting out if not

Re: Code 33
 

A laptop would be terriffic but I don´t think I can get hold of one in my neighborhood. I found a vacuum guage and hooked it up. It reads 14 inch hg and steady at idle. So back to the facts;

1. Engine stalls on occation in the mid range. Especially when the engine is warmed up
2. The ECM throws code 33 repeatedly. If I clear the codes and start and stop the engine it will throw 33 again.
3. Engine vacuum at idle is from what I can understand normal, 14 inch hg.
4. MAP output at idle is with both MAPs that I have tried 2.7-2.8 V which is from what I can understand way too high. Even at sea level.
5. The troubleshooting procedure points at the MAP.

Should I get hold of another MAP? I am not 100% sure that the second MAP was OK.

GOODT, do you know what the MAP output should be for 14 inch hg?

Is it possible to get a small leak at the MAP mounting hole that messes up the MAP reading?

Re: Code 33
 

If the volts are high it means low vacuum signal, check for a leak or clog to the MAP sensor, possibly a clogged port in the manifold.

Re: Code 33
 

I have now made some investigations and calculations. The GM 1 bar MAP sensor is a linear device 0-1 bar. 13.7 inch hg vacuum should give a MAP reading of 2.68 Volts for the old MAP. It reads 2.67 so there is nothing wrong with the sensor output.

Is 13.7 inch hg vacuum at idle good enough?
Why does it throw code 33?
Why does the engine stall?

Re: Code 33
 

Okay, I like your thinking, so let's look at this a different way. The MAP is reading right but setting a code when the engine stalls because vacuum goes too low. So what else could cause stalling? If you are not loosing fuel pressure it should start right back up. If you are losing spark you should see the tach drop out quickly. You may have a sticking idle air valve if it only stalls at idle. Have you checked the basics-cap, rotor, wires, fuel sep?
What is base timing and full advance?

Re: Code 33
 

I have been doing some reading and I am beginning to think there are two different problems. The code 33 is triggered because the MAP output voltage at idle (=TP sensor less than some 2-4 %) is too high. If the MAP output is correct, it means that vacuum is too low. This could be because the throttle plates do not close completely assuming there are no vacuum leaks. This I suspect because I have been adjusting the throttle plate opening to get them fully open at WOT. Maybe I tightened the throttle cable too much.......

The other problem is the stalling at midrange which most likely is ignition related. The tach drops faster than the rpm when it stalls.....

Re: Code 33
 

insptech. What drives the tach? Is it the pulses on the primary side of the coil or is it something on the high tension side? I am trying to understand why the the tach drops faster than the rpm

Re: Code 33
 

the tachworks off of the dist pulses at the - side( primary ) of the coil

Re: Code 33
 

in most cases a mid to high rpm range stall is fuel related,

I think you are trying to over engineer the problem go back to basics, as Ted stated check fuel pressure ect

Re: Code 33
 

Hi . Thanks for your input. Keep the ideas coming.

Fuel pressure is OK, rock steady through all the stalls. The tach drops faster than the rpm and if the tach is as you say GOODT driven by the primary side of the ignition that is where the problem must be. Somehow the squarewave on the primary side cuts out, taking away the input to the coil as well as the tach. I have checked the coil and the magnetic pick-up in the distributor. Most likely it is an open circuit or a short somewhere....

Re: Code 33
 

mats,
Have you checked or replaced the ignition module? It should be mounted with heat transfer compound and it needs a good ground to the distributor.

Have you cleaned and checked the connections at the back of the module (where the magnetic pick-up connects)? That is a real trouble spot, especially with the added moisture associated with MSD ignition. I can't remember if you have MSD or not.

What type of plug wires are you using? I have heard about some strange problems due to EMI on computer controlled engines with aftermarket plug wires.

Dave

Re: Code 33
 

Hi Dave. well I have not checked the ignition module but the whole distributor assembly is new. Is there a way to check the ignition module or do you need an oscilloscope? The plugwires are stock. I have checked them with an ohmmeter and they seem to be OK. I have checked the connection on the back of the module and they were corroded but they seem to be OK now.

Question; if the tach wire(s) gets shorted to ground, can that bring down the whole primary squarewave?

Re: Code 33
 

basicly yes and it will stall the mtr

Re: Code 33
 

They do sell inexpensive module testers, but I don't know if they will really tell you if you have an intermittant module failure. A scope would be a much better way to test it. A test bench where you can test the entire ignition system would be even better :rolleyes:

Yes, it sure will.

Dave

Re: Code 33
 

I have seen similar problems with a failing module. It may be bad, even though the dist is new, especially if it wasn't installed with the heat transfer compound. There is also a possibility that the short wiring harness off the module has a broken wire. A good shaking on all the wires around the distributor might turn up something also.

Re: Code 33
 

Guys, I think I found the problem. One of the tach wires on top of the engine , in a wire loom close to the engine harness, shorted to the engine. She still throws code 33 but now she runs fine. This engine is driving me nuts.

Dave, GOODT, insptech, thank you so much for your assistance.

Mats