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Discussion Starter #1
This one has me stumped, and now I'm starting to shotgun things, which isn't free. Maybe one of you guys has a clue.

I have a GM 350 (5.7L) EFI in my '97 box truck, with 110K miles. Starts great, idles great, oodles of power... but... once warmed-up, on very slight partial throttle during level driving and mild downhills, I get hesitation, and "soft" backfiring through the exhaust. No engine codes.

First attack was replacing distributor/rotor and plugs even though neither was very old (15K). This smoothed shifts and improved mpg about 15% (yay!)... but the hesitation got worse. Plug wires were new at that last tune-up, and I've been adding Techron roughly every 5K since then, "just 'cause".

I'm thinking a mixture management issue, and the backfiring sounds like a lean-out from my old carb days. Next on the shotgun list are O2 sensors, but that's getting into some serious coin. So... not having a full-blown analyzer system in the garage, any thoughts?
 

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Check the EGR Valve.

Had one of the computer controlled ones go bad on a mid 90's 3800 V6 and it caused the same symptoms. EGR only operates at part throttle.

Just my 2 Cents.
 

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do you have the old plug wires? if you do try them. look at the ignition coil.(Idont know if it's in the cap or not on that year)check to see if it has a purple haze around it or purple spots that look like it has got hot (the spark is leaking out ) if it has any sigines change it. yes the egr could also do this.also a vacume hose with a leak can do it. do you hear any hiss around the dash?possiable heater controal door hose leak. or fresh air.controal.have you changed the gas cap lately?if si it might be the wrong one. fuel pressure is also something to look at.(&fuel pressure regulator & vac hose) or might just be burning a valve ,also sounds like dead camshaft but with a roller cam that dosent happen.(I think that should have a roller lifter cam).good luck
 

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On my old '95 GMC, there was an ignition module underneath the rotor - looked like a flat piece of gray plastic with a metal backing. It's screwed to the top of the distributor housing with a couple of screws and they intended it to get a good ground that way. Trouble came when they started using an aluminum backing on the module stuck against the dissimilar metal which causes it to lose the good ground over time. (it's very common on GM) You pull off the module, clean the back with steel wool, and then stick it back on again.

My mileage would jump about 2mpg and the engine ran a lot smoother too. I've seen others that wouldn't even run at odd rpm's for the same reason.
 

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Mike that's an injection issue. You would need to start with a test of the sensors.

Let me see if there is information on the web. I found it at one point. Then was unable to locate it the next time I needed it.

Try this site. Not the one I was looking for. The one I was looking for is Free. Still it's less expensive than changeing out all the parts one at a time.

Dom
 

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Discussion Starter #6
Thanks, guys... I don't feel so bad now... four responses, four different answers. Neat stuff.

All good clues - armed with the EGR idea I found several forums talking about flushing the EGR with carb cleaner, so that's a quick'n'cheap, at least as a diagnosis tool. The common problem appears to be carbon buildup causing it to stick as opposed to outright failure.

Plug wires are certainly a thought, since I obviously fooled with those changing the plugs. OTOH a weakness there would also tend to show up under WOT or any heavy load, and no problems. Same goes for ignition module ground, although freshening that is free. And a good idea.

IAC? Good thought. We're talking about ever-so-barely-open on the throttle conditions, so the IAC might be in play. That the idle itself, though, is silky smooth and even makes me wonder.

I also need to double-check the MAF. If it's a vane type (this part could easily be different on the Isuzu given its odd intake setup), a sticky MAF reluctant to budge under low-flow conditions could be messing with things.
 

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what about a vacume leak?

or maybe an auto trans shifting/overdrive issue....that just feels like an engine thing?
 

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UUUhhhhOOOOOO

There is a good chance that you have an Intake manifold crack, if so it would get worse when warmered up. hope not that job is a Biznitch-L, you can check it with a ciggerette, just hold a lit cig around the intake and watch where it go's, " the fan is gonna blow it around, but if you have a leak you will see it" good luck, gotta love a Chevy 350
 

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wd 40 to check for vac leak ,spray & see if the idle changes. the module needs a dieletric compound between it and the dist, to help with heat. tps, etc.good luck. my bet is on the wires or burnt coil. but sure sounds lean
 

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good idea dan. also on moters that are droping a cylinder you can when there cold start it up for 10 to 20 seconds sometimes longer depending on type of ex manifold. & check each exhaust port/tube/manfiold/at the cilinders with the engine now off. the "dead cyl "wont be hot but the others will.
 

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Discussion Starter #12
The winner in the category, "Best Performance in Diagnosing 'Mild Hestiation & Backfire Under Partial Throttle'" is...

...envelope, please... thank you...

...the star of EGR Valve, our own steamloco76. Please step up to the podium and receive your trophy, a slightly used but perfectly serviceable EGR valve gasket.
Valve pintle had a slight stick, which cleared up with cleaning. It runs great! I love 10-minute fixes on stuff you can get to with two bolts.

Thanks to everyone for the assistance with thinking this through.
 

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Wait! Hold on.....


I'mma let ya finish but I think Marksbug really deserves this award.......blah blah blah
 

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I'll share-:-D

I was an EGR problem and also a gasket problem.

The vehicle that taught me that answer had a carbon chunk stuck in the pintle seat.

Glad it was a cheap fix.
 

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Discussion Starter #15
... had a carbon chunk stuck in the pintle seat.
Ah! That makes me happy I ponied-up the extra few bucks for the gasket with the built-in screen. Apparently it's to prevent exactly that problem.

Seriously - thank you.
 

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wd 40 to check for vac leak ,spray & see if the idle changes. the module needs a dieletric compound between it and the dist, to help with heat.
Doggone it but my face is red. I forgot to mention the grease but it's kind of important...
 

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Discussion Starter #17
A revisit for the gearheads...

The problem came back when temps warmed-up, so apparently we were close, "but no cigar". Did more research, and the possible causes were still all over the map. One was easy, and the symptoms sounded too familiar - ran fine except under the slightest touch on the throttle. Advice was "check TPS" for linearity.

Bingo.

Turns out that the GM TPS can develop a little rough spot just off of idle due to dirt accumulation. As a result, this simple 5K potentiometer will have a small resistance spike that will "lie" to the ECU, telling it that there is less throttle open than at idle, which then leans the EFI even more.

Solution was beyond easy. Remove TPS (two T25 screws), spritz the insides with DeOxIt, turn a few times to move the dirt out of the way, reinstall, done. No more backfire.
 

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well duh I thought you would of already of done that first !! that's why I never said for you to do it every body knows that !!:shock: and dont forget to clean the iac.and lube the muffler bearings :razz: just kidden, glad you got it figured out. ( but the gm Iac does need cleaning,brake kleen,keep water away from it and the tps they dont like water.
 
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