CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Any GM specialist out there?

I’ve created a discussion a long time ago about this truck and I really appreciate the past help, but I’m back for more.

My 2001 Chevy S-10 with a 4.3L V6, has finally decided to bite the dust, but I’ve been trying to revive it. The problem I’m having now is that it is hard to start, but once it does it idles perfectly. Not only this, but while it’s in its “warm up” mode (where it idles at about 950 RPM) the engine will accelerate, hold any RPM and purr like a kitten, but as soon as it gets out of this “warm up” period (where it idles at about 650) the engine will not accelerate. It only spits, sputters, tries to run up and, if I dont back off, will finally die. If I do back off, though, it will return to idle and idle perfectly. Any attempt to press the accelerator again will cause it to spit and carry on like above.

Here’s some additional info: It’s throwing the P0300 “Random-multiple misfire” code. I’ve replaced the following parts: Fuel pump–fuel filter–fuel injection unit (spider)–fuel pressure regulator–coil–distributor cap and rotor–plugs and plug wires. Here’s the things I’ve checked: fuel pressure 50-60psi–spark at the plugs O.K.–borescoped the #1 cylinder and the valves are opening and closing on the correct strokes–removed the catalytic converter and muffler (running open pipes) so there shouldn’t be any blockages–lastly, the throttle position sensor (in accordance with a web-site) it checks out O.K. I’ve also put a hand-held scanner on it and if this strikes your fancy, I’ll provide all the diagnostic readings it gave me.

Any suggestions, direction, advice…or matches? Thanks a million.

Is it possible the cam or crank position sensor is in your list of diagnostic readings that are out of range? Not positive this motor has them. Could it be something in the air bypass valve circuit or even the EGR circuit that is causing a problem?

It has a crank and cam position sensor, but i’m not sure if the scanner picked up any problems there (unless it’s hidden in something like "LT FTRM1 % = 7.8).

Also I pulled the EGR valve off and looked at it (sorry, forgot about that one) and it didn’t seem to be faulty, i.e.: stuck open/ closed or covered with carbon.

I would be wondering about the MAF sensor. Make sure all the plumbing between throttle body and MAF is properly hooked up and no leaks. If my memory serves me right, you should be able to electrically disconnect the MAF and if it’s the problem it should run a LOT better. A long term fuel trim of 7.8% is just fine. I don’t worry about the fuel trims as long they aren’t in the double digits.

Was it idling poorly before you went to open pipes? I’m thinking this reconfiguration might have affected the upstream oxygen sensor readings. This would make sense, because when the engine is cold the ECU ignores the O2 sensor reading in order to allow the engine to run rich, and when it warms up the sensor loop is then added to the equation.

Are you losing coolant? Maybe coolant is leaking into the intake when the engine is warm and coolant is under pressure. When the engine is shut off coolant would dump into the intake and cause problems starting but once started it might blow off the coolant and run well until heated again.

Soryy about the delay, had to go back to work. To answer all questions:
–I disconnected the MAF sensor and there was no change. MAF scanner readings are:.85 lbs/min @ idle and 1.76 @ 1200 RPM (freeze data for P0300 code)
–It wasn’t idling poorly before I went to open pipes, actually it idles very well all the time. I haven’t ruled out O2 sensors, here’s the reading: O2S11 (V), O2S12 (V) and O2S21 (V): .1 to .8 (all #'s varying the same @ idle) either the scanner didn’t provide data or I forgot to write it down for the freeze data, sorry.
–Coolant levels appear to be normal and the oil level isn’t above normal or milky looking. Are there other methods to check for this? I’ve had the intake plenum off several times and there is no evidence of water there. Any tell-tell signs I can look for in the cylinders?

Thanks

I just read one of the other discussions about thier PCM problems. Are there any DIY checks for the computer? Are the PCM’s from different cars compatible (same year, model, ect of course) so that maybe I can do a troubleshooting swap?

Also, I don’t know if this has anything to do with my engine problem or not, but when my truck was drivable, the cruise control worked intermittently and sometimes the tranny wouldn’t shift into OD.

I’m not a technician but during open loop it uses preset information from the sensors. Once it goes into closed loop, it is using reading from the sensors instead. Seems to me this is your major clue, that a sensor is not providing a good reading, not able to, or the PCM is not recognizing it (I don’t know how likely this is if the PCM is working). To me you need to just get it on a diagnostic computer to see what the sensors are providing to the PCM to see what is out of kilter. A random misfire I believe is the result, not the cause. As far as the PCM goes, they have a serial number on them and any computer with the same serial number will do BUT you have to use the prom chip from the current one in the new one. I used to care a spare computer that I paid $25 for at the junk yard just to rule out computer problems. In 20 years, never had any.

The scanner I have tells me the information the sensors are sending. Is there a website or book that I can reference the numbers to so that I can identify the faulty sensor? I’ve asked a couple of local dealers about the readings and all I get is “bring it in and we’ll hook it up to our reader… for a price”.
Most people tell me the same thing about the PCM–they rarely go bad and when they do the car won’t start at all. I don’t know enough to have an opinion on the PCM. I wish it had a carb and magneto. These I can work with.

Update by the way, the death rattles have begun…the truck wont accelerate past 2500RPM now and the sputtering is worse. But it still idles fine.

Try posting the sensor numbers here, the experienced scanner users can probably tell you if they are in range. And, no charge, heh, heh.

Yeah that or the ranges would be in the factory service manual or probably Alldata. Some of the readings though are not static like the O2 sensor.

Silly question here, are all the ground straps in place and making clean contact? Thought process being as the RPMs come up so does the amount of current available from the Alternator. With grounds weak or not present the signal floor/ground may not be stable enough to read properly and cause offset voltage levels at the PCM.

Here are the readings:

At idle------- Freeze data on P0300 code

MIL Status--------ON
ABSLT TPS (%)-----0.0 ABSLT TPS (%)---------5.9
RPM------------615 RPM----------------1183
Calc Load %------3.5 Calc load (%)---------7.8
MAF (lbs/min)----8.5 MAF----------------1.76
MAP (inHg)-------11.2 MAP---------------11.5
Coolant----------152 deg Coolant-------------172 deg
IAT-------------88 deg
Ign Advance------16-20 deg (fluctuates)
Second Air-------ATMOS
ST FTRM1 (%)— -.87 to -2.5(fluctuates) ST FTRM1--------- -.8
LT FTRM1 (%)---- 7.8 LT FTRM1--------- 4.7
ST FTRM2 (%)— -.8 to -2.3 (flucts) ST FTRM2--------- 3.1
LT FTRM2 (%)— -7.8 LT FTRM2--------- 4.7
Fuel Syst 1 % 2----Closed Fuel Syst 1&2------- Closed
ST FTRM11 (%)— 0 to -3.1 (fluct)
O2 S11(V)-------- .1 to .8 (fluct)
O2 S12 (V)------- .1 to .6 (fluct)
O2 S21 (V)------- .1 to .8 (fluct)
ST FTRM 21 (%)-- 0 to -3
PTO stat--------- dise
OBD2 stat------- CA

I’l check all the grounds I can get to today. I’ll also check batt voltage, static and running, and see what I come up with.-Thanks

I think I found the problem.
While looking around, I decided to check the distributor and rotor one more time. The rotor looks as if it has burned through. There’s a black burnt spot leading from the finger and goes all the way through the plastic that holds it. This thing was replaced at about the same time I replaced the fuel injection unit when I was chasing another hard starting problem (before it quit me on the road this last time). I suppose this was giving me my hard starts and P0300 code, and now that it’s burnt through, could be causing the truck to not run above idle. I plan on buying another tonight. If this fixes it, I’m going to feel really dumb…

Nothing to feel dumb about just gleeful that you found the problem.

Story: When I quit smoking for a while, I wanted to clean my daily driver completely. I took the bucket seats out to shampoo the carpet etc. When I put them back the car wouldn’t start. Tried everything and couldn’t figure it out. Had it towed to the shop and they found an ECM fuse missing. How it happened I have no idea, but when I said how dumb I felt about it, they told me don’t feel dumb, it took our best mechanic over an hour to find the problem. Simple stuff can be just as hard to find as hard stuff.

Well, it was as simple as that. Replaced the rotor arm and she cranked like she was new. I had one of those “I know I checked that!” moments and then, was like you said, just glad I fixed it.

Thanks again, to everyone who posted—I really appreciate all the advice and help.