Chevy & GMC Vans

Hi, All - I recently purchased an '05 AWD Astro that is bone stock with ~ 150K miles, and am in the process of making it reliable for use as a daily driver and adventure wagon. To date, I've done the following:

Replaced spark plugs, plug wires, rotor and cap
Cleaned mass air flow sensor with MAF cleaner
Replaced air filter
Replaced transfer case fluid with GM Auto-Trak fluid

The problem I'm currently chasing is that the engine runs very rough under mild acceleration from stop and at low-speed (50-ish mph) cruising, i.e., off-idle and at light load. It starts easily, idle is fine, and it seems to have plenty of power under more aggressive acceleration, including at wide-open throttle. It's only in the mid-range of throttle setting and engine RPM that it misfires. The misfiring seems random, and significantly reduces power. It never backfires. It's throwing code P0420 "Catalyst System Efficiency Below Threshold" and P0300 "Random/Multiple Cylinder Misfire Detected". The description of the P0420 codes says that a cylinder misfire could cause this code to be thrown, so I'm not sure how much credibility to give to the P0420 code at this point.

After reading through many of the problem descriptions in this forum and the great advice given by many of you, I haven't seen a description of a problem that's quite like mine. It's still not clear to me where I should be looking for the root cause. I don't want to shotgun it and replace a bunch of parts unnecessarily, so some advice would be severely appreciated.

Here are some things that I think are suspect - which ones do you think are the most likely culprit? Are there others?

One or more bad O2 sensors
Bad throttle position sensor
Bad ignition control module
Bad ignition coil

Thanks!

I bet it could be TPS.
Good luck. My 2003 AWD has 125,000 miles. No problrm yet. Thaks for your good info.
AP

Or what about a bad temp sensor?
Can you run your scan tool in data collect mode while driving? Then it may show more sensor data.

AP

Astrophysics wrote:Or what about a bad temp sensor?
Can you run your scan tool in data collect mode while driving? Then it may show more sensor data.

AP

Unfortunately, I don't have a scan tool that's capable of real-time readout (yet!). I cleaned the MAF sensor and temp sensor again this morning, and am planning to pull the ignition coil and ignition control module off this evening for test. Not sure how much credibility I give the testing done by AutoZone. It appears to be more of a go/no go test on the coil, rather than an evaluation of the health of it - but you never know. Same for the ignition control module. Since it has ~ 150K miles on it, it might be prudent to just replace them.

My '03 is now at 171k.

I've had to replace the plugs, wires, distributor cap, rotor. I was still misfiring some time later, come to find out the distributor itself was worn out and needed replacing.

I was getting some catalyst codes also. Replaced some oxygen sensors, cleared the codes and never received a repeat.

Just make sure you use the crowfoot with the O2 sensors. It was a PITA for me without!

If you're still having issues after the ignition has been rectified, perhaps a fuel delivery issue? How old is the filter?

Ken

I've now replaced the ignition coil and ignition control module. It made no difference in the way it runs. I bought a coolant temperature switch and throttle position sensor and will install them in a couple weeks.

After reading more posts on this forum, I'm now beginning to suspect the O2 sensors. I looked at them yesterday and found three - two in the exhaust pipes between the engine and catalytic converter, and one behind the catalytic converter. All three look OK as far as being in one piece and having wires connected. Wires look OK. Is there a way to test the sensors without pulling them off the vehicle?

Thanks!

Before you swap out anything else, go get yourself a couple bottles of GM Fuel System Treatment Plus. The P0300 is a fairly common occurrence with dirty injectors. I've had it on a few of my vans. (Currently hunting for my 8th Astro.) Regular fuel injector cleaner didn't cut it, but the heavy duty GM stuff did the trick every time.

ihatemybike wrote:Before you swap out anything else, go get yourself a couple bottles of GM Fuel System Treatment Plus. The P0300 is a fairly common occurrence with dirty injectors. I've had it on a few of my vans. (Currently hunting for my 8th Astro.) Regular fuel injector cleaner didn't cut it, but the heavy duty GM stuff did the trick every time.

Thanks - it's worth a shot. I'll pick some up tomorrow. I'm assuming the dealership is the only place to get it?