95 Astro, intermittent stalling, now dead

I had intermittent stalling at all temperatures, when at idle, acceleration, and deceleration, with no change in fuel pressure and no fuel odor. When I would rapidly press then release the accelerator pedal, there would be a fairly loud electrial crack, which seemed to come from the front of the engine, possibly on the driver's side, and I could not see any sparking in the dark. Now, the van will not start.

Here is a list of what is not wrong:
-fuses
-fuel pump
-fuel filter
-injector unit
-cap and rotor
-coil
-ingnition module
-pickup coil
-ECM

No Codes have been set.
What's left? MAF sensor? MAP sensor?

1995 AWD, 4.3L:banghead: W

I will be in and out all morning, so I might not respond very quickly.

Before the responses come flooding in, I should probably add that the oil pressure has always been excellent.

Always be sure that a part is faulty before you replace it, guessing gets expensive quickly. The very first part that you replaced may be faulty. Secondly, just because a part is new, that's all it means, it doesn't mean it's good. You asked, "What's left?” You haven't even scratched the surface of possibilities that can cause a no start condition. Your very first statement, "intermittent", may weigh heavy on what the problem turns out to be. Usually "intermittent" is traced down to a loose or poor electrical connection. Lets start with some basics.

What is the fuel pressure with the key on and the engine off while the pump is priming? What brand of parts did you use? When were the spark plug wires replaced and what brand did you use? When it cranks do you have a blue, (hot) spark at more than one spark plug? If there is a problem in the OBDI system, it should set a code and turn the check engine light on. If the light is not on, we're most likely looking for a fuel delivery problem or an ignition problem, neither system is fully monitored by OBDI. Post your results, and we'll continue.

The parts I've listed above are parts that I've replaced from time to time within the last year and had tested yesterday. I've inspected electrical connections and made sure that the components have the appropriate voltages. The fuel pressure with the key on and engine off is 63 psi. Blue spark jumps an inch from each plug wire. The cap and rotor are AC Delco OEM as are, I believe, the plug wires. I am not replacing parts on a hope and a prayer that I find the problem; rather, I've been systematically eliminating the possibilities, beginning with the most obvious first.

I mention the MAF sensor because I know it provides information related to fuel delivery, and I mention the MAP sensor because it was suggested as a possible cause for engine stalling in a post some time back. As for testing the MAF sensor, I do not have access to a frequency meter for testing the unit.

If you need any further specific information let me know. I'll be right back, because I have to pick the kids up from school.

The MAF sensor can cause stalling without setting a DTC, (diagnostic trouble code). However, if the MAF sensor and/or its circuits cause a no start condition, it would most certainly set a DTC. The same holds true with the MAP sensor. If the TPS voltage is near 4.5 volts, the injectors will not operate while cranking.

Approximately 3 seconds after the key is turned to the RUN position, (engine off) the fuel pump should shut down. Fuel pressure must remain above 55psi for several minutes after the pump shuts down. If it does, the next step is to remove the upper air filter housing from the vehicle. Leave the MAF sensor and air inlet ducts intact. Give the engine a quick shot of starting fluid into the air inlet duct and immediately try to start the engine. Will the engine run, at least momentarily?

-TPS voltage is 4.7 volts
-Fuel Pressure stays well above 55psi for several minutes--it varies
-Introduction of starting fluid or gasoline does not cause even momentarily
starting of the engine

The engine did start for approximately 3 seconds prior to testing for the above, but now it won't start at all, again; although, it has acted like it wanted to start on two of the last ten attempts--chugged. I even swaped out the filters, just in case but, alas, to no avail.

Still no codes set.

Soon, my floor will be carpeted with the hair I've ripped from my head, and my teeth will be ground level with my gums.

If TPS voltage is 4.7 at idle, that's a problem. Reference voltage should be close to 5.0 volts, (grey wire). Signal return should be below 1.25 volts, (dark blue wire). The black wire is sensor ground and should be 0 volts. 4.7 volts is not out of the TPS's range so no DTC would be set, that explains no check engine light.

The TPS voltage is 4.7 volts with the key on, but not at idle because the van wouldn't start. I used another volt meter and it reads 4.8 volts. The voltage for the signal return is approximately 1.2 volts.

The engine started and ran for about 3 seconds 10 minutes ago, but it won't even act like it wants to start now.

I have to take a break now because I have to feed some kids, and it's getting dark. But, I'm not ready to quit just yet.

The 4.7 volt reading you found is reference voltage and is correct. The TPS (signal return) reading you found to be 1.2 volts is correct. The TPS is not the cause of the no start.

You mentioned introducing starting fluid had no effect. You also mentioned that later it ran for a few seconds and quit. I would start looking for a loose or poor connection in primary ignition, crankshaft position sensor or possibly the ignition switch.

It's 15 degrees and dropping, with snow on the ground, so I'll have to wait until tomorrow before I crawl under the van and check the connection on the CPS. Is there any way to test the CPS without using a duty cycle meter? and, can one test the Mass Air Flow sensor without a frequency meter? Actually, how likely is it that the MAF sensor could cause this problem? If the ignition switch were faulty, wouldn't there be other systems affected at the same time? And lastly, if the electrical connections are okay, what would the next step?

I do appreciate the assistance you are giving me. :grinyes:

I crawled under the front of the van to check the connection on the CKP sensor, and lo and behold, there is no sensor nor was there ever one. Instead of a sensor, there is a soft plug crusted with years of road grime, so I guess that can't be the problem. All of the electrical connections look good, with no apparent corrosion. But, I am reluctant to tear into tear into the steering column, partly because my son ran off with all of my pullers and partly because all other systems still function--even codes set when sensors are disconnected and the engine is cranked.

Now for the latest, my van finally started and is currently idling and running great. I'm going insane trying to figure this out. :screwy:

When I first started the van, it ran rough and made a loud sucking noise through the intake, and it did this for about 15 seconds; then, the engine coughed and both the idle smoothed out and sucking stopped. I've been driving it around a little, afraid to venture very far from home. I've tried both rapid and slow acceleration and deceleration, on and some off road, and turning it off and on, but it still runs great. I don't know which way to go! :uhoh:

Just one more thing, the electrical crack I mentioned in my fisrt post can no longer be heard when pressing and releasing the accelerator pedal.

Welcome to the world of intermittent diagnosis! The old adage of "If it ain't broke don't fix it." takes on a new twist, "If it ain't broke you can't fix it." This is not acceptable, so, the adage is modified, "If it ain't broke, mess with it and it will break." While the engine is idling, very gently grasp each sensor wire close to its connector and wiggle it. If that brings no success, take a small, (6") screwdriver and gently tap on each sensor.* Be very careful to "mess" with only one thing at a time. Patience and perseverance pays off when it comes to intermittent problem diagnosis.

* MAF sensors have been known to have intermittent internal problems.

I've wiggled, riggled, and rapped, but it still runs smoothly, with no stored codes yet.

About two weeks before the germlins started stalling out my engine, the transmission began to shift hard through all gears, though, hardest going into second and third. Since the engine began running again, the shifting has smoothed out once more. Might that be a clue to the cause of the intermittent stalling?

The TPS and the MAF both play important roles in transmission shift points. Since the TPS can cause a no start condition, and not leave a DTC, lets check the TPS again, a little more thoroughly this time. With the DVOM, connect the negative probe to ground, connect the positive probe to signal return, (dark blue wire, the one you found 1.2 volts on). Turn the key to the RUN position, (do not start the engine) and slowly move the throttle to wide open, then back to idle. You'll be watching for a smooth increase and decrease in voltage. Voltage should increase to near reference voltage, (5 volts) at wide open throttle, then decrease to near 1.2 at idle. Repeat this procedure a few times until you're sure the change is smooth, no spikes or drop outs in voltage.

Here is the latest. When testing the TPS, the voltage increased and decreased, from 1.2 to 5.0 volts, smoothly with no spikes or drops. After over 200 miles of trouble free operation, the grimlins struck again. My engine stalled 1 1/2 miles from home and stalled almost 30 times, for the next 45 minutes, before I got home with the van, and, unlike previously, the shifting was still relatively smooth. Again, I tested the TPS, and this time there were voltage drops, which seemed to occur consistently at certain voltages. The only stored code was a 23 for the IAT, which was caused from driving that 1 1/2 miles home with the IAT disconnected, so I cleared the code 23 and tried to set a code for the TPS by slowly depressing and releasing the accelerator pedal while the engine was running over a 10 minute period. During this 10 minute period, the engine ran rough with a very slight surge occuring every second when idling, and white smoke billowed from the exhaust; even though, the engine was at operating temperature and the ambient temperature was 38 degrees. Of course, no codes were set. So, I tested the TPS again, and it tested perfect. When I started the engine, it ran smoothly, and it no longer belched white smoke; although, this time, air duct and IAT were connected. I now have nearly 30 miles on it, and it still runs great. The only thing I messed with during all of this, other than disconnecting the battery to clear the code 23, was the TPS and air duct.

By the way, there is no MAF sensor, unless it is cleverly hidden--CMFI.

...Again, I tested the TPS, and this time there were voltage drops, which seemed to occur consistently at certain voltages. ...So, I tested the TPS again, and it tested perfect....

If the TPS voltage remains in range, (above .5 volts and below 5.0 volts), a DTC will not be set. The processor does not classify voltage drops or spikes that are within range as a fault. It's up to the technician to determine if the spikes are abnormal. Your first test showed voltage drops, and that's a problem. Your second test had acceptable results, much like your 200 miles of trouble free driving. The TPS is faulty, replace it. Cross your fingers and hope that's the only problem.

For those who had been following this thread, the TPS does appear to have been the problem. A couple of hundred more miles and the problem seems a thing of the past, but it ran well previously then crapped out on me. Hopefully, this thread has come to an end. Thanks for the help.

Knock wood it's fixed. Intermittents can be a real bear at times, but it looks as though you've pinned this one down. Persistance pays off. I don't want to be premature but, glad I could help. If it acts up again, post back.