'97 4.3 Runs bad under 2500 then perks up

[TMcKissick]

A month ago my very beloved Astro caught a bug. It runs horrible below around 2500 RPM in all gears and has less power and mileage above 2500. Occasionally it clears itself by launching forward in the middle of 1st gear very powerfully. It stumbles terribly and backfires out the exhaust. Runs good for the first 30 seconds after a cold start though. I'm getting good at keeping it at the top half of every gear to avoid the pops but I can't hold speed on the highway without downshifting unless I'm above 75 mph. I've checked about everything I can think of and am out of suggestions.

I get a multiple cyl misfire code and one for inefficient cat (but that one's been there for 6 years with no problems other than setting the check engine light). I've replaced the plugs (couple yellowed, couple a little dark, one perfect, one pretty wet), and wires, fuel filter and some vaccuum hoses. I replaced the cap, rotor and plugs about 5 months ago after a MAJOR misdirected cap troubleshooting event. I replaced the MAF then also on a bad recommendation for no change. That time I just felt I needed a tune up because of lost mileage and roughness on the highway. Now, I've checked the TPS (smooth ramping up and down), air temp sensor is in range, ignition module was load tested at Autozone, all 3 O2 sensors bounce rapidly +/- .2 to.4 around the .5 volt mark, EGR (cleaned, leak tested, disconnected), and the map sensor voltage (all sensor checks were made by backprobing the computer connector) is in the middle of it's good range. I can't find a true engine vaccuum line but a check valved one sticks at 18" and doesn't drop until the idle is increased to 1500 very rough in gear. Fuel pressure is 57 at idle and varies from 54 to 59 at times.

Getting frustrated. Help!

[TMcKissick]

I forgot to mention. The van's got 325,000 miles on it but it still starts, runs and drives very well.

[jdmccright]

I've seen a string of the similar problem with '96 and '97 GM trucks lately with CSFI-style injection. Under the plastic upper intake plenum there is a central fuel injection "spider" that delivers fuel to the intake runners via plastic poppet valves. These tend to get clogged and stuck. The cheap route would be to give it a good soak cleaning to free up the poppets. Or you can replace the whole spider assembly with one that places the fuel injectors where the poppets were. Here is an example of the conversion:

http://www.theautoshop.net/VortecMPFI.htm

The fouled plugs mean you've got leaking seals, this will also cause misfiring and the cat problems (they may be shot due to all the unburned fuel going through them). You may also have carbon build-up in the cylinders and valve stems, causing predetonation. Grab some cans of Seafoam and treat the engine, oil, and fuel...use as directed! Then change the oil after 500 miles.

Then I'd check the plug wires for any cross-firing or arcing to ground, the distributor gear for excessive wear, and the intake for any vacuum leaks. A good cleaning of the MAF sensor can't hurt either. Hope this helps!

[TMcKissick]

Can it really be dirty poppits if it perks up at around the same RPM in any gear. It seems that more load (hills, headwinds) moves that RPM up higher. I mean it really smooths out nicely. Also, it runs perfect when first started cold for about 30-40 seconds regardless of immediate driving or just idling in neutral. The main reason I question injector problems is that I've had 2 major problems in the past and everyone jumped on the injectors right away and it wasn't them either time. I ran injector cleaner through them a few weeks ago and that didn't change the problem at all and I always use non-ethanol gas.

I've checked for vaccuum leaks with both WD-40 and choke spray and I've replaced the plugs and wires with no change.

The distributor has a little bit of up-down play. You can hold it one direction and lift about .200" or so and feel the pitch of the cam gear. It's not enough to allow me to pull the cam sensor without pulling the distributor though. There's no noticable side to side play.

How does one clean the MAF sensor? I've always gone on the premise that choke spray, ether or any other cleaner will kill it. Also, this MAF is only 5 months old (the routine tune up back then) and it's output to the computer is in line with the Hanes recommendation.

Could this all be ANOTHER bad distributor cap? or cam sensor?

[TMcKissick]

Had to jump back on here and post an update. After 2 months of troubleshooting (and driving) this problem with everyone's advice, I kept having this nagging feeling that it actually could be a 2nd bad distributor cap. I was complaining about that to someone yesterday and found that they had delivered one to my house while I was at work (probably to just shut me up). I put it in and vio`la... Problem fixed!

I can't convey how ticked off I am at this company for cheap crap parts. This is the second time I've had a less-than-a-year-old distributor cap from them go bad in an odd way. The last time resulted in over $1,000 in parts (tows, diagnosis, fuel pump, coil, crank sensor, knock sensor, plugs, starter - my fault, etc.) and this time was over $200. On top of that, my first alternator from them lasted only 3 days before locking up. My first starter from them had bearings grinding and went back immediately. My front brake rotors from them are still warping (with no drag) in 15,000 miles which is about 6 months for me. They've ended my free lifetime replacements (after 5 replacements - but they won't warranty the idler arms that THEIR rotors tore up which are their cheap 1 year pair) now so I'm going with Napa or GM this time. Oh, and one thermostat that didn't want to ever re-close. I'm now wondering how long my half shafts are going to last. I replaced them on yet more bad advice when my transfer case viscous clutch froze up and caused the chain to pop in hard corners.

All in all, except for not standing by their TSB for paint delamination on the hood at 6 years and 6 months (6 months over), I have to say that this Chevy Astro has been pretty dependable. Actual problems have only been an alt, water pump and transfer case clutch. The rest of the problems have all been because I replaced perfectly good tune up stuff at 200,000 miles because I figured it was just time and used crap parts. Not bad for 324,050 miles.

[jdmccright]

Thanks for letting us know you found the problem, just sorry we weren't able to save you from the headaches.

All I can mention is that regarding the brake rotors, be sure the calipers are working smoothly (pistons and slide pins) and that the rear drums are adjusted properly so that the fronts aren't doing all the work when you're braking.

Congrats!

[TMcKissick]

Originally Posted by jdmccright

Thanks for letting us know you found the problem, just sorry we weren't able to save you from the headaches.

You're welcome and thanks for the input.

Originally Posted by jdmccright

All I can mention is that regarding the brake rotors, be sure the calipers are working smoothly (pistons and slide pins) and that the rear drums are adjusted properly so that the fronts aren't doing all the work when you're braking.

Congrats!

Been there, dun all that. Filed the caliper sliding surfaces flat and smooth-er. Tried dry and lubed. Don't feel any drag at all. Replaced and adjusted back brakes (although once again, they didn't really need it because I have driving to work down to only using the brakes about 6 times). One interesting quirk that I can't figure out is that after driving 40 miles in the rain and not touching the brakes or cruise the whole way, a slight touch on them makes the vehicle dive to the right. Just the one time. That's why I filed the slides smooth to get rid of even the surface rust. No change.