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4.3 running rough ... again

4.3 running rough ... again

Postby jml755 [OP] » Sep 30 2012, 6:51pm

I've chronicled my '02 Silverado engine problems on other threads. Got about 125k on it. I've had varying symptoms and problems such as hard starting, especially when wet. Then when I got that fixed, it started to run rough and missing. I'd be on the expressway doing 70 and all of a sudden it would bog down, start backfiring and I couldn't get above 45 mph even at WOT. First time it happened, I limped home and turned it off and after several test drives, restarting of engine it was still backfiring like crazy and no power. Then I checked the oil and was a little low, so I put about half a quart in, fired it up and it ran fine. WTF? That went away for awhile and it started again this summer. It got to the point I was afraid to take the truck on anything other than short trips I ended up replacing the Crank sensor, the cam sensor, TPS. Over the last year, I've changed plugs, wires, coil, rotor, distributor, MAF, FPR, Fuel pump, fuel filter, you name it. Had to take it to a shop after replacing the crank sensor for a reset. I can't believe all the massive problems I've had with this engine. It's a spare vehicle so I doubt I've put more than a couple thousand miles on it in the last year. It may sit for a week without being driven.

Anyway, its been running ok since summer after I replaced a bunch of stuff and then started running a little rough last week. Not real bad but wouldn't idle rock steady like it had been lately and a little hard to start. Then, on Saturday, it happened again. On the expressway at about 70 and it just starts bucking, misfiring, backfiring out of the blue and can't get above 45mph. Pulled over, checked the air filter (which is almost new). Took it out and tried again. Still couldn't get over 45. Pulled over again, checked the oil. It was a little low so I topped it off with about half a quart. Started it up and voila ran fine the rest of the way to my destination, no backfiring or anything. WTF? Heading for home after it sat for 3 hrs it started a little hard and was running rough until it warmed up and it was fine the rest of the way home (30 mile trip, mostly at 70 mph hiway). Got home and it would start but run rough at idle. Today, same thing and I put a scanner on it. The HO2 sensors are supposed to be between 200 & 800mv readings. Both Bank 1 & 2 # 1 sensors (before the CATs) are 30-50 mv while the #2 sensors (after the CATs) seem ok at around 190 &350.

The question I have is: does this mean my # 1 O2 sensors are probably bad? This isn't a tough fix and relatively cheap ($ 60 or so for the part).
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Firing on 4 cylinders [L1]
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Re: 4.3 running rough ... again

Postby Roscoe-6 » Sep 30 2012, 7:06pm

The oil has to be a factor here. Is it possibly losing pressure causing the malfunction? In my experience if it's the o2 sensor all its going to do is throw a code at you causing your engine light to come on. Either the oil is burning or leaking somewhere internally or externally.
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Re: 4.3 running rough ... again

Postby jml755 [OP] » Sep 30 2012, 8:26pm

The oil pressure is around 40 typically and will drop when it's running rough and also get up to 60 during revving of the engine. I suspected that it could be oil pump originally, thinking that maybe at a lower oil level the pump isn't lubricating the engine properly, creating friction, causing the engine to lag, etc.

But its still running rough with the oil topped off now and the # 1 Ho2 sensors are reading way below their normal values.

As a disclaimer, I have been running with codes 171,174 (lean condition) and the static fuel trim is above normal (which is 23%). But it's been running like that for years and the lean condition has been attributed to a probable lower intake manifold vacuum leak most likely. Guys on this forum have said that the 4.3's are prone to those. I have changed all the gaskets on the UIM and throttle body. My point is that I've had the 171,174 codes for years even when the truck has been purring like a kitten and they don't seem to be related to the obnoxiously rough running of the engine.
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Re: 4.3 running rough ... again -update

Postby jml755 [OP] » Oct 07 2012, 4:41pm

Well, truck was still running rough last Tuesday. I took it for a short test spin and it was sluggish, hesitant on acceleration, slow increase of throttle would hit 2500 rpm, drop down to 2000 and go back up as pedal was still being depressed, same thing when it hit 3500. Idle was not smooth and releasing the throttle at 2000 rpm would cause it to drop down to almost 0, sometime stalling.

Planned to replace the almost NEW Fuel filter, if that didn't help, maybe change lower intake manifold gaskets (vacuum leak?), injectors on Saturday (yesterday).

But on Tuesday, before I went back inside for the night, I took off the MAF, sprayed it with MAF cleaner, reseated the air filter and tightened the bolts on the air filter housing (remembered one was loose and could be removed by fingers). Didn't restart the truck until yesterday.

Yesterday, before I went to the store for a fuel filter and other $$$$ parts, I started it up and it was idling smooth. In neutral, letting up on the accelerator at 2000+ RPM and it would drop to 700 and stay there. WTF? Decided to forego any more work for now and I dumped a pint of Seafoam I had into the tank and took it on a 60 mile round-trip (mostly highway). It ran like a top, good acceleration, smooth operation.

The MAF sensor is brand new (< 1000 miles on it), one of the many things I've changed. The scan values are slightly lower at idle than before and don't correlate to the truck manual. My cheap scanner says .5 lb/m (pounds/minute) which is over 13000 grams/sec. The manual spec says it should be under 655 g/s, IIRC. But the values have always been around .5 lb/m, running rough or smooth so I don't trust the scanner.

I am still getting a P131 & 134 code indicating lean operation but it hasn't set the MIL light yet, but the 171 & 174 codes aren't there (yet, fingers crossed) like I was getting before. Fuel trim is still higher than it's supposed to be so I suppose I could still have a vacuum leak on the lower manifold or injector issue. Everything else in the fuel system has been changed (pump, FPR, filter).

Also, the HO2S sensor readings are pretty close to each other so I think the CATs are probably going.

So, it could be the MAF cleaner did the trick. Anyone ever see a problem fixed by tightening the air filter housing bolts?

I'm hoping I've got it resolved, I can live with a MIL and throwing lean codes as long as it runs smooth and I can depend upon driving it without it almost breaking down on the expressway.
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Re: 4.3 running rough ... again

Postby ASTE25 » Oct 07 2012, 4:48pm

Just for safety sake, take the distributor out and check to see if the gear or shaft is worn. It's a long shot, but I have heard of a worn distributor shaft causing problems. The oil pump is driven by the distributor in your engine. That is about the only connection I can think of between the oil level and your drivabality problems.
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Re: 4.3 running rough ... again

Postby country1911 » Oct 07 2012, 4:55pm

I am guessing your fuel trim being off for a while has fried your cats. I would solve all those issues with trouble codes prior to doing anything else.
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Re: 4.3 running rough ... again

Postby jml755 [OP] » Oct 07 2012, 11:06pm

If it starts acting up again, I'll probably pull the lower intake manifold and do the gaskets. Pretty cheap ($30 or so) plus my time and that would resolve any vacuum leaks at the manifold which I've hear guys say is a common problem. Would also be a proactive fix for any potential coolant leaks which I understand are common on that engine as well. Checked all the other areas for vacuum leaks and there were none. I'll also pull the distributor and check for wear.
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Re: 4.3 running rough ... again - update - fixed

Postby jml755 [OP] » Nov 25 2012, 2:46am

Well, the truck had been running decent since last post until a few weeks ago.Then it did the same thing (with no warning while running on freeway at 70, just bogs down and won't go over 45, backfiring, missing). Since then, I've been driving it locally a mile or so at a time and noticed that it's basically running poorly under load (starting in 1st gear especially), it will bog down and backfire, buck like a bronco, but when I revved the engine and slipped the clutch a little, it would pick up speed. Revs were not a problem without a load. Figured it was fuel related and since I'd changed everything else, I got an MFI injector "spider" setup to replace the oem SCFI . Ran it this AM for a few miles to make sure it was still having issues, pulled it in the garage and did the job (under 3 hrs). Took a test drive and it purrs like a kitten and there is NO hesitation on acceleration or under load in 1st gear!

Also, the engine had been running lean, throwing codes with Static fuel trim running way past max of 25%. After I put the new MFI in, no lean codes and fuel trim is around 18-24%. I've got my fingers crossed that this thing is FINALLY fixed.

BTW, the kit that I bought said to replace the fuel body bracket (not included with the new injector setup) because the old one wasn't compatible. The bracket cost "$30" !! The old one had 3 clips on either side to hold the fuel body which only has 2 retaining tabs. The 3rd clip must be for some other application. The new bracket had only the 2 tabs needed for the fuel body. I'm pretty sure you could use the old bracket and just use an angle grinder to cut off the front section where the 3rd retaining clip is. Since I already bought mine, I just used the new one. But if you've bought the kit and read the instructions (which mention the incompatible bracket) on the day you're doing the job, grab the angle grinder before you waste time heading back to the parts store.
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Re: 4.3 running rough ... again

Postby Bigbob66 » Jul 02 2014, 8:32am

Do yourself a big favor and read up on timing marks for the engine and get it to TDC#1 cylinder. Mark the distributer, since the rotor moves clockwise during installation. Now, remove the distributor assembly and carefully inspect the drive gear. My drive gear had 3 teeth sheared off and had worn quite a few of the others. It was doing the same thing as yours. I would recommend replacing the whole assembly if worn. Finally, since the vehicle has been running rough, a new cat may be in order. Mine had plugged up with rust from the resonator and completely melted the internals of the cat. I only learned this stuff after replacing all the intake manifold gaskets, a plethora of parts, and after replacing the dizzy, it would start and run for a bit...then bog down. WOT would make it worse...and eventually, she would just die. The initial problem was the dizzy gear, but the cat was really the culprit. Good luck!
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