no power under load + backfires - fuel pressure? ignition? sensors/ecu?

[brush]

Well, it is exactly 180 degrees, but with the odd number of gear teeth it results in your ign. timing being a half-tooth too advanced or retarded no matter how you insert the dizzy, with not enough range in the advance adjustment to bring it right. This can happen if you rebuild the dizzy with either the drive gear or the mechanical advance mechanism wrong way around.

hmmm, the issue is that the distributor hasn't been moved since before this issue arose, when it was working fine. is there a way to tell whether this might be a problem, without disassembly?

you do bring up the mechanical advance again, and remind me of that issue. there has been a debate about whether the 2nd gen trucks have a mechanical advance. when i connect the jumper that cuts out the computer for the timing, there is zero advance with increased revs, even with engine racing. if anything, it very slightly goes towards zero/to the right, about 1 degree or so (i think this might just be that everything's moving so much faster).

can someone 100% confirm whether their 2nd gen truck shows any advance with the jumper set?

that would be amazing, thanks!

[brush]

you do bring up the mechanical advance again, and remind me of that issue. there has been a debate about whether the 2nd gen trucks have a mechanical advance. when i connect the jumper that cuts out the computer for the timing, there is zero advance with increased revs, even with engine racing. if anything, it very slightly goes towards zero/to the right, about 1 degree or so (i think this might just be that everything's moving so much faster).

can someone 100% confirm whether their 2nd gen truck shows any advance with the jumper set?

hi folks! one last bump on this. i really can't think what else it is, but throwing more hours (and potential problems) on swapping the distributor when actually it's fine would be pretty painful. i might throw in the towel if i can't get this.

i feel like someone doing a check on a 2nd gen with a timing light to see if the mechanical advance moves with revving, with the "ECU cutout" jumper set, would help not only me but others.

thanks!

[brush]

well, urgent other projects left the truck sitting for a while, but i'm back at it. i removed the crank/cam position sensor from my donor distributor (the 1996 mighty max), with the idea that maybe just swapping sensors would be a great place to start, as a failing sensor would explain the weird jumpy timing light as well as ignition problems. however, the connector for the 1996 is different from the connector on the 1991. does anyone know more about this? also, it appears to be difficult-to-impossible to find the sensor (is it cam or crank or both?) for sale online. anyone know about that? part name?

i guess my main question is: are the sensors compatible? ie. if i cut the wires and splice to the old connector housing, should it work?

(fwiw, rockauto lists the same distributor for both 1991 and 1996.)

thanks!

[brush]

some updates and requests for help:

after some research, i think crank position sensor is integral in these 2.4 89+ distributors.

so i broke down and swapped the whole donor distributor in. as others before me, had some challenges getting it in the right tooth. i *think* what i realized is that you have to align the rotor to #1 plug at TDC *when the housing is in fully-retarded position*. if you try doing it when the alignment bolt is at the notch (which is close to where the base timing will be), then you easily get a tooth off.

unfortunately, the situation is unchanged with the new distributor. it times to 7deg BTDC when the notch is just about aligned with the bolt. however, backfires and carbonizes the sparkplug. when maximally advance the distributor, it runs much better -- but the advance is way off.

on another forum got the suggestion that advancing timing has side-effect of leaning mix, so perhaps rather than a timing issue this is just a symptom of an underlying running-rich issue. again, having (i think) ruled out the fuel pressure regulator, and with good compression readings, what could be causing rich mix?

thanks folks!

[brush]

hi everyone! i have some updates.


spoiler: i've tuned it so that it's got 80% of its power back, with only quite rare little backfires! here's how we got there--


i tested the vacuum with a mityvac i have around, and the results were interesting. the main "symptom" is that the needle flutters very quickly back and forth about 1 inch Hg. this *could* be that the gauge doesn't have a damper, apparently, but also might be useful info.


otherwise, at idle and warm the vac is around 17-18 inches. not great, but not bad afaik. rev quickly and release, and it drops to zero then back up quickly to around 23, then settles back. so -- no clogged cat. maybe worn rings. rev high (no tach, but in vicinity of 3k rpm by ear), and it goes down on accel, stays level around 18 or so when even revs, then goes up a bit when decel. seems normal. otherwise, no big issues (eg. spontaneous drops in vac, etc.)


so. if the flutter isn't an artifact, then i think it could mean one spark plug is underperforming, or a valve is a bit tight or got friction or something.


so i start to explore the sparkplugs. bingo - the #1 plug is more carbonized that the others. testing the gap, it's also a lot smaller -- like .030 when the others are .039, which is spec for that plug. (maybe the #1 got bumped a lot as i was pulling it over and over to get TDC correct.)


however -- spec for the vehicle is .044! i check, and the original plugs before i put in new ones are also at .039, so it's been running with subpar spark the whole time.


gapped the sparks, and there's a distinct improvement in performance. can accelerate up hills, much less bucking and hesitation on accel, and much fewer backfires (which only happen at low rpms under load).


so this is exciting!


after gapping the sparks, the flutter in vacuum is less but still present. when i rev high, it sometimes goes away completely, but then back at idle. unlike previously, it seems as if it's mostly at the high point, then once per cycle (ie. per crank or cam rotation) it drops about 1 inch. this makes me think maybe one of the plugs is still acting up, or maybe a valve has problems.


anyone have suggestions for how to further explore this?


at this point, with how well it's now running at correct 7 deg BTDC timing, i'm thinking the timing is no longer a likely cause of remaining problems.


(in related news: on the suggestion above, i thought to clean the MAF. pulled it and sprayed with CRC MAF cleaner, waited for it to dry, then put it back in and it seemed to make things better! wow! THEN i read the fine print that said not to use the MAF cleaner on karman vortex MAFs like in the mitsubishi -- it can destroy them! argh! but things seem fine so far, knock on wood. lucky i have a spare from the other vehicle.)


again, thanks for everyone's input! appreciate further thoughts!

[geezer101]

*don't use platinum plugs either. Use the recommended rated plug or slightly hotter rated plug to fend off misfire by burning carbon deposits.