Manager Forum

Well I find myself again in need of the expert advice of my fellow techs out there.

I have a customer in here with a 1998 Saturn SL2 4 cyl. automatic transmission having 140k on the odometer. Here is where we stand:
Customer brought car in for a condition where when she stops at a store, or something that only has her away a short time, when she returns, the car would not start. Finally could get it started and then drove on.

I inspected and found a missfire on #4 cyl, and vehicle would overheat, with gauge showing normal. I replaced the #1/4 coil due to no spark, and the ECT due to inaccurate signal (Common problem I have heard) Also we took this time to replace her original plugs and wires. Customer left happy, engine was running fine.
Customer has now returned for a stall at stop lights. Test drove and found that engine would cut out and or stumble when initially tipping in the throttle. You could drive through it if you gave it third throttle or more. TPS sensor shows that all is within reasonable specs. At idle TPS reading is 0%, and .35 volts. Reading increases steadily as you press throttle. No codes stored, IAC system tested ok. Any ideas?

First, how did you test the TPS? Are you just reading scan data or did you test it with a scope? Most scanners will not pick up a glitch. Also, how did you test the IAC? And what are the IAC counts at idle when warm? What is the fuel pressure and the fuel trims? How does the throttle body look?

Throttle body looks good and clean. IAC was tested using the scan tool running it up and down. IAC counts I do not recall exactly what they were, but they were around 40ish. Fuel trim was at -7 LT and 0 ST. Fuel pressure was 45psi at idle.

And how about that TPS? Any chance of getting that waveform on a scope or at least a Vantage or other graphing multimeter? (scope would be better)

40 sounds a little high for the IAC, but I'd be more concerned with low. Is this a fully warmed up stall or a colder stall? Did they have this stall before? What kind of plugs and coil went into it? (all these questions, eh? ) Does this have a fuel pressure regulator? I'm not at my info system right now. Have you monitored fuel pressure during a stall? Would like to rule that out.

RESULTS OF INCORRECT OPERATION
The results of incorrect operation with too much EGR flow at idle, cruise, or cold operation, any of the following conditions could occur.


Engine stops after cold start.
Engine stops at idle after deceleration.
Vehicle surges during cruise.
If the EGR valve should stay open all of the time , the engine would not idle. The results of incorrect operation with too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This could cause:


Spark knock (detonation).
Engine overheating.


I work on these beauties from time to time. I kind of agree with Ric about the t.p.s. but i've seen bad e.g.r's cause similar issues. I know it's linear valve but i've seen them flutter and cause issues. The skip and overheat could have been caused buy a stuck e.g.r. and carbon. The failing t.p.s. could send a false signal to the pcm and force a false e.g.r. signal.

The overheat was DEFINATELY caused by a faulty ECT. The ECT data showed 85 degrees when my temp probe was showing 250 degrees. Cooling fans never turned on due to faulty ECT sensor. Vehicle no longer overheats. I do not know if the stall occurred before, because when she brought it to me, we couldn't even get it to start, and once we got it running (after replacing coil and plugs) we found the overheat condition while it was warming up in the stall. Fixed overheat condition and drove it. At that time it didn't stall, but I will admit I have a heavy foot, and if you DO heavy foot it past 2% throttle, it runs just fine. When she drove it for me, she accelerated just tipping in the throttle (Data shows average of 5-8% when she hit the pedal) and at that point is when it would stall. It will do this hot or cold. The fuel pressure is fine at stall out. The car will immediately restart, and again, if you just throttle past 10% it takes off fine.

Customer finally had brought the car in, and as a courtesy I replaced the TPS for her. Engine is still doing the exact same thing. Stalling out when you tip in the throttle. It will do it in or out of gear, but more prevalent in gear. (under a load) I have no codes, and to be honest, I am a bit confused on this one. Not that I have never ran into problems that were strange and been able to fix them, but that with the data and things I have inspected, I can't seem to find a logical REASON for it to stall. It's not even a stumble/stall. The engine just QUITS like you turned off the switch.

I've had a few cases where engine movement caused a problem. A few were cracks in the air inlet tube on MAF vehicles. Also had a few where a wire was rubbing to ground and needed a certain movement to short.

One other thing. I found some pattern failures that match this when you accidently cross #1 and #4 wires. Check spark plug routing. Engine will run fine like this, but will stall just off idle. Please check........

I crossed the wires once I think it was this engine it didnt stall but missed and took me forever to figure out. Of course it was a friday and late to

Ric, Bryan, you both hit it on the head. When I replaced the coil I accidentally got the two wires mixed up on the coil. Damn I am kicking myself! Cost me a TPS sensor too! Thanks tho. Always glad that there are so many people here who can help. Just goes to show you, even the good techs can sometimes make a simple mistake.

You're welcome.

I'm not sure why it's critical. Something to do with cam sync I think.