OT 2007 GMC truck running issues

Hey guys,

We have a mechanical issue we can't seem to solve..

Working on a 2007 GMC w/ 4.3 V6

Idles fine when cold, once warm, will not idle a majority of the time.
Has major miss on cylinders 2, 3, and 4 between 1200-2000 RPM. Have not checked it at higher rpms, at idle (when cold) none seem to show miss.

The other 3 cylinders show occasional miss in that rpm range (our scanner "resets" the miss counter every 30 seconds or so, cyl 4 will show anywhere from say 40 up, 2 and 3 will usually be about half of that number, and the others ma show 1 or 2, if any)

What has been done:

Few months ago, we put a cap and rotor on it.. Has ran fine since until about a week ago (but has only been 2k miles since changing) and has ran fine until about a week ago..

Checked fuel pressure.. Shows 60 key on, truck off.. 60-65 truck running at various rpm's. Slowly drops after key is turned off.

We swapped plugs around (took #4 plug and put it in #1 cylinder) the miss did not change.. stayed at #4

Checked for vac. leaks (ether trick) Nothing there.

Checked for wire arcing.. Nothing we can see.

We removed cap and rotor, just to have a look.. Looks good there.

Cleand the MAF sensor (with MAF cleaner, per directions on cleaner can)

Today, we took the coil/ICM to a parts store, and they tested both, said the ICM was bad.. $75 later, no change so to speak.

We have changed the temp sensor as well (at a previous date) and the argument between my father and I is that the temp sensor likely has nothing to do with it

Previously dad changed the temp sensor, as it showed a "code" about low temperature- which I can see a logical problem with a half stuck open thermostat, which we also changed previously.. I don't believe the sensor was bad when he changed it, and I can't say one way or another, since we changed both at the same time. I say it's not the problem now because A) it shows accurate temperature from gauge to what our scanner says, and B) NO temp code this time.

Anyone have any ideas what to look for at this point?? Our scanner will tell us different outputs of different sensors, but I don't know what "spec" would be on most of them to know if there is a problem there or not.

Thanks for insight in advance,

Brad

I've had three high miler 4.3's we had to file a bit off the base of the distributer. And rotate the dist the old fashoned way to get the crank and dist sensors to agree.
One of those 4.3's could never control temps about 1/2 way through it's life. Several new stats of every brand and yes they were installed the correct way up. The engine would get super hot then the stat would stick in the 1/2 open position.
I kind of suspect a manifold leak, egr leak or other air entry after the MAF.

I don't know the cyl. numbering for your engine but we've had problems at work with a bad cat.converter for one bank on a V-8 that messes with 4 cyls. at a time. Usually get a MAF sensor code when it happens.

Been there,done that.

Throw a GM cap & rotor on it.Haven't seen an aftermarket one that worked right more than a month yet.Everytime it rained or was foggy,they either missed,backfired,or didn't start at all.

Is this the model with the cap that has the crossover tower contacts? I have heard about caps failing allowing one cylinder to spark 100% of the time. It has to do with aftermarket caps cracking internally and the center coil jumping contact to that one cylinder. The others are starved for spark. Try using a timing light attached to each wire and see if on of em is going nuts.

Aaron

I'm with bradk. If You got an aftermarket cap and rotor go back with ac-delco one. Had the same problem with mine when I changed out to a supposedly top of the line autozone model! About 3-4 months later the problems started up. Spent days trying to figure out the rough running/miss firing problem. Went to a s-10 club website and they pointed out problem right away! Upstart is I went to dealer parts counter and spent about 15 bucks less for the stock parts and they work perfectly!

Scott

I'm with B&D on this one. I'd look for a intake manifold leak. Not sure if the 4.3 uses a plastic intake like the 5.3's. Had a 5.3 that had a bad intake manifold gasket and it also would not idle once warm. Ran pretty decent cold. Just my $.02

Gotta say ,,great help request ! Covered basics, lots of info . Now Like Brad said ,,GM cap n rotor. If still giving you fits , possible intake leak ? I know you mentioned you checked it , but ,I've seen them leak on the bottom of gasket , no external vac leak . Might pull oil filler cap and give quick shot of carb spray see if smoothes out . what are your Fuel Trims doing ?

I vote for AC Delco cap and rotor BTDT

Guys,

Thanks so far..

I talked to my dad, and he don't recall what kinda cap he bought (which means it was CHEAP).. I put a NAPA cap on my 4.3 about 10k or so ago and it has been fine.. But again, I know dad didn't buy a NAPA cap either.. My dad believes a NAPA store with a higher price than the "big box part stores" just want to rip you off.. Personally, I have had far better luck with NAPA parts than anything bought from Murray's/ O'Reilly's, Auto Zone, ETC..

We'll try the timing light on the plug wires, and that should help tell a little more on what is going on with the misfire. We were going to swap plug wires to see if that made a difference tonight, but was mom's birthday, so we went out to dinner.

I also think trying the carb cleaner in the oil cap sounds like a good idea as well.. I don't think it has a plastic intake, but did not pay that close attention.. I guess I never thought something like that would cause a miss, but it would affect the vacuum, which in turn affects other things..

Dad does not recall what the fuel trim was showing, so we will check that tomorrow and post back.. He's supposed to be the one that works the computer, but sometimes neither he nor I know what exactly to be looking at.

And Bryan, when it comes to info, I'd rather throw a whole bunch out there in hopes to get the issues resolved than get 100 responses telling us to do what we already looked into.

Thanks again, will post back tomorrow.

Brad

Don't bet on a NAPA cap being of high quality,, ECH trade mark has always been brass terminals used on their caps,,,if it does not have brass terminals its a $2 cap in a $50 dollar box... BTDT the echlin cap was junk and I knew it when I removed it from the box,,, echlin sold their sole to the chines...

One week later and I have to tow it in it runs so bad... I jump my napa guy about it and he admits ech made the switch and its a on going problem... I don't trust ech anymore...

UPDATE:

Pulled the cap this morning and returned it to O'reillys. They had a Delco cap, so we got one of those instead of a cheap piece of no name garbage.

Came home, put it on, and it seemed to cure the issue.

Thanks guys, cause i would never have thought a nearly new cap would have been the problem. Told dad no more no brand junk parts.

Brad