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

[brush]

hi everyone,

i originally posted this under the 2nd gen thread, but i think that was the wrong place.

i'm working on a '91 mighty max 2.4 for a friend, and it was running super rough, hard start, etc. i did lots of things, swapped a bunch of parts with a '96 mighty max parts car, and finally swapped the entire exhaust manifold/catalytic converter. this fixed the main issues -- i think it was a combo of failed spark wire and warped manifold/busted studs leading to clogged cat. however, during the process i changed the distributor timing, and didn't have a timing light so had to set it by feel.

things were a lot better, but still having very weak power at low RPMs or on hills (under load). i suggested not to drive it much until i got a timing light, but i think it got driven at least a hundred miles.

got the timing light, and the timing was way advanced, i think. like 15-20 degrees (!?!). i was jumping the wrong jumper at first, so that was the post-computer timing. now, with correct jumper, the timing is a steady 7 deg BTDC. however, with jumper ungrounded, it jumps around from ~9 degrees to ~14 or so. not smooth, each rotation is a bit different. not sure if that's normal?

anyway, now it starts well and everything is a lot smoother -- but still getting pretty terrible power at low RPM under load, and also backfires. not huge ones, put pretty frequent coughs/puffs of smoke. i drove it for ~10 miles to see if the computer just needed to reset, but not much change. disconnected battery, cleaned connections, and reattached (ie. reset ECU), no apparent change.

at idle (which is more or less ok, though it wanders), when i open the throttle, the RPMs go *down* and it bogs, almost cuts out, unless i feather it. sounds to me like fuel isn't getting added when throttle opens, right?

disconnecting the TPS or the MAF sensors appears to have little or no impact on the problem. i *think* this means they're not likely culprits. i'm not hearing misfires, so that makes me think spark is less likely the problem.

so: fuel, right? what's the best way to attach a pressure tester to these trucks -- i know some folks have had a hard time with it? the only other thing that makes me doubt fuel is that the truck was running fine before the spark/exhaust manifold issues cropped up, and it would be strange if a completely unrelated thing bust as well... yeah?

other thoughts?

appreciate the help!

[xboxrox]

Does the distributor have a vacuum advance? Test the vacuum diaphragm actuator see if it holds vacuum & advances the timing? Remove air cleaner cover with engine off look down the carburetor while opening the throttle you should see gas streams from the acceletator pump (does it have a carb?)

[brush]

thanks for the response!

it's a fuel injected engine, no carb. not sure if there's an analogous way to check how much fuel's coming out of the injectors.

i don't *think* there's a vacuum advance, but i'll check tomorrow. but your point makes me wonder: maybe there's a problem with the mechanical advance in the distributor? especially since the problem shows even when i revv the engine with the car in neutral -- it bogs down unless i feather it. then when i get to high rpms (like, maybe the mechanical advance unsticks??) it seems to be better.

what's the best way to test this? i do have this '96 parts car sitting here, would it make sense to swap distributors and see if that changes anything, or is there something else that would be easier?

thanks again!

Does the distributor have a vacuum advance? Test the vacuum diaphragm actuator see if it holds vacuum & advances the timing? Remove air cleaner cover with engine off look down the carburetor while opening the throttle you should see gas streams from the acceletator pump (does it have a carb?)

[geezer101]

4G64 doesn't have a vacuum advance - all controlled by the ECU. You need to do some basic checks. First see if you are getting ANY codes from the ECU that indicate a fault. The ECU's themselves are notorious for failing but are not a nightmare to fix or diagnose. Cracking the ECU case open and a visual inspection usually reveals bad capacitors or worse case scenario bad tracks on the PCB. If the ECU has failed, it will operate in 'limp home' mode which means it'll run, but not be happy about it.

[xboxrox]

To check the mechanical (or any 'n all advancing going on) would'nt it be easiest if you used a timing light while revving the engine..? Locate & put a big blob of white on the pully timing mark so it's easy to see... I used my wife's fingernail polish pen with white color...

Actually, I don't know if my timing idea will work on a fuel injected ECU engine, I assume your motor has a distributor, maybe not..? I would just remove the dist cap and try moving the base plate parts that move when the mechanical advance is activated... Could be things are stuck or in need of cleaning & some lube... Try not removing the distributor unless that's the only way to fix it, too much work in my opinion... Sometimes we break more than we fix, I do, so it often pays to do only what's needed & no more... I have a 50/50 success rate...

Hoping for your Success
George

[brush]

thanks for both responses! a few updates and replies:

first, i swapped the ECU with the '96 computer, and no changes in the symptoms. so i *think* that rules out busted caps.

second, i checked the codes and just get a neverending quick blink, which i believe means no codes. same as last time i checked, before the ECU was reset.

third, i believe that the timing does not really advance cleanly when revved, but i'm going to do it again with better lighting. so that *would* tend to imply the mechanical advance?

finally, that's a good idea about working with the baseplate parts. i took out the distributor from the parts car, but having a heck of a time getting the wiring connector phillips screws to unstick. so if i can avoid moving the distributor that's better.

any particular hints on messing with the base plate?

thanks!

To check the mechanical (or any 'n all advancing going on) would'nt it be easiest if you used a timing light while revving the engine..? Locate & put a big blob of white on the pully timing mark so it's easy to see... I used my wife's fingernail polish pen with white color...

Actually, I don't know if my timing idea will work on a fuel injected ECU engine, I assume your motor has a distributor, maybe not..? I would just remove the dist cap and try moving the base plate parts that move when the mechanical advance is activated... Could be things are stuck or in need of cleaning & some lube... Try not removing the distributor unless that's the only way to fix it, too much work in my opinion... Sometimes we break more than we fix, I do, so it often pays to do only what's needed & no more... I have a 50/50 success rate...

Hoping for your Success
George

[xboxrox]

I used a driver drill with the best fitting + driver tip I could find to remove carburetor screws that my bare hands could'nt budge... Start with low setting then keep increasing until the screws finally come loose.
You'll get it

[SubGothius]

Get a JIS* screwdriver or bit set. Hozan or Vessel brands are good and widely available online. I like my ball-grip Vessel with swappable bits, even came with Pozidriv bits that came in handy for assembling Ikea stuff.

Philips screws/drivers are designed to cam-out as a torque limiter for ease of assembly without stripping threads, but this can lead to stripped heads with repeated dis/reassembly or trying to undo even a mildly-seized screw.

JIS screw heads look similar but differ, being designed for a more secure fit between the screw head and driver bit; these often (but not always) have a dimple on the head, so that's a dead giveaway you've got a JIS screw, which is pretty much all of them on these trucks. A Philips driver bit engages these even more loosely than a proper Philips screw head, further contributing to head strip-out.

Nearly anything assembled outside the US will be using JIS or equivalent screws, so having the proper driver/bit will help enormously. Better yet, a JIS driver bit also works better on actual Philips screw heads, and a JIS screw head stripped by a Philips driver/bit may still be recoverable with a JIS driver bit.

*Technically superseded by DIN 5260-PH/ISO 8764-1, but outside the industry ppl still call them JIS anyway as they're nearly-enough identical, and only these newer successor standards are readily available anymore anyway.

[xboxrox]

We should all have a set of JIS drivers ~ thanks SubGothius

[brush]

this is super helpful, i didn't know about JIS screws, explains all the challenges i've had with TPS screws, coil, etc. with various vehicles. usually end up putting hex-head screws back in!

i've been pretty busy, but getting back to the project now. i did check the timing with the ground-wire connected to turn off the ECU, and it was absolutely steady no matter how much i tried to revv it. revving is hard -- if i open too much, it gutters and drops lower, but if it feather it slowly and carefully i can get it to what sounds like 2500+, with no advance.

so that's definitely a problem, right?

i will say i liked xboxrox's idea of working on unsticking the advance before swapping dizzie's (with potential alignment problems, stripped screws, etc.). but my initial look at it doesn't make it obvious how to do this. any ideas? has anyone had an experience like this?

fwiw, it looks like the PO replaced the distributor recently, it's shiny-looking, and it could be a cheap-o part.

also still would love to rule out fuel pressure as a problem, anyone have a good way to test? (how *do* you attach a pressure gauge to these things? <shakes head>)

thanks again everyone!

Get a JIS* screwdriver or bit set. Hozan or Vessel brands are good and widely available online. I like my ball-grip Vessel with swappable bits, even came with Pozidriv bits that came in handy for assembling Ikea stuff.

Philips screws/drivers are designed to cam-out as a torque limiter for ease of assembly without stripping threads, but this can lead to stripped heads with repeated dis/reassembly or trying to undo even a mildly-seized screw.

JIS screw heads look similar but differ, being designed for a more secure fit between the screw head and driver bit; these often (but not always) have a dimple on the head, so that's a dead giveaway you've got a JIS screw, which is pretty much all of them on these trucks. A Philips driver bit engages these even more loosely than a proper Philips screw head, further contributing to head strip-out.

Nearly anything assembled outside the US will be using JIS or equivalent screws, so having the proper driver/bit will help enormously. Better yet, a JIS driver bit also works better on actual Philips screw heads, and a JIS screw head stripped by a Philips driver/bit may still be recoverable with a JIS driver bit.

*Technically superseded by DIN 5260-PH/ISO 8764-1, but outside the industry ppl still call them JIS anyway as they're nearly-enough identical, and only these newer successor standards are readily available anymore anyway.

[brush]

quick update: took off distributor cap, and attempted to turn rotor. does not move, except some wiggling of the rotor on the spindle. am i correct that rotor should be able to be rotated by hand, on the order of ~20 degrees or something, and then spring back when released? does that happen on your working truck?

[xboxrox]

My 2.6L with Mitsu distributor the advance plate moved quiet a bit of movement too & with just pushing with my finners <g>

[SubGothius]

quick update: took off distributor cap, and attempted to turn rotor. does not move, except some wiggling of the rotor on the spindle. am i correct that rotor should be able to be rotated by hand, on the order of ~20 degrees or something, and then spring back when released? does that happen on your working truck?

That would be correct if your dizzy has mechanical advance, but AFAICT from the exploded diagrams in Haynes, looks like the '89-on FI engines had no advance (vac or mech) built into the dizzy at all.

[SyrsKnight]

to me it sounds like its jumped time or somehow gotten itself out of time for one thing. The backfiring is a huge give away on that.

[brush]

thanks, SubGothius, that is super helpful. i also tried turning the distributor rotor on the parts truck, and it doesn't turn either, so unless they both are rusted tight -- i guess there's intentionally no advance? as for the timing being off-- what i think is going on is that the timing was set way advanced previously, and it was kind of working but had low-RPM problems (which kind of makes sense). i then timed it "properly" with a timing light using the numbers on the plastic housing, and it's now got the problems i describe: terrible loss of power when attempting to revv, and backfiring. which i think makes sense if either a) it's not advancing enough with increasing revvs, and/or b) it's leaning fuel (due to pressure or other things). (am i thinking this correctly?)

so -- *could* it be a slipped tooth on the timing belt? which might mean that my timing light-setting is way off? would there be other symptoms?

or could it be that some sensors (which ones?) are causing the computer to send completely wrong advance info? (when the computer is engaged, the timing jumps around randomly within 5 degrees every rotation, it seems like, not sure what that means.)

or, of course, could just be low fuel pressure maybe? claych offered to talk to me about how to test that (thanks), but any thoughts are welcome.

[camoit]

Post up some pictures. They all advance and they all should turn. Something doesn't sound right.

[claych]

Post up some pictures. They all advance and they all should turn. Something doesn't sound right.

I must disagree ,a fully electronic engine control system with cam angle sensor(S) and a crank angle sensor
configuration does not use or need input from a mechanical device...

I also would like some pictures -- Because I do not know every thing & weirder engineering has happened
Thanks

[xboxrox]

claych ~ do you happen to know if any years of our trucks have all those angle sensors ? Hope the guy's distributor don't get broke trying to force something that don't supposed to move

[camoit]

As far as I know. None of theses trucks had crank or cam sensors. It was before they came out. Unless someone put in a different engine. This is why when setting the timing you grounded out the plug to keep the ECU from adding advance to the mechanic advance.
once the cam and crank sensor came out we did away with the distributor

[SubGothius]

No crank sensor needed when the dizzy pickup for electronic ignition is already providing that signal (x4); then it can be static timed without any mech/vac advance, and all advance can be determined by the ECU. From there it was a small leap to eliminate dizzys entirely in favor of crank sensors and directly ECU-triggered spark.

[claych]

Apologies for the confusion, xbox
I should have been more concise.
I used the word 'configuration' in post seventeen and should have differentiated
between separate individual sensors mounted at the crank & camshaft(S)...
And a single optical 'sensor' mounted in the distributor that provides
a crank and cam position reference signal.(4g64 M/M R/50 , late '88-early '95).

brush? an update?

[brush]

delayed with all the holiday stuff. need to get back on it though. i don't want to bust anything by trying to force it either! how do i post pics on here, can't see how to do it?

[brush]

ok, i'm going to try and update with photos to address this question of whether the distributor is at fault.

this photo is of the distributor in the '91:
https://photos.app.goo.gl/rBxFc89ofqSY789y5

you can't see it well, but it's there and a bit shiny, indicating it was fairly recently replaced by previous owner.

these are pictures of the mostly removed dizzy in the '96 parts car, which i think is identical, and also doesn't rotate when i hold both ends (though that's harder to try):
https://photos.app.goo.gl/GmhMdf7jTjFyE8JU8
https://photos.app.goo.gl/b6YuwfJ37TyFvGQYA

back in the '91 here is a poorly lit video of me trying to rotate the rotor, unsuccessfully:
https://photos.app.goo.gl/SF1gFVQ9hedaJq3H7

whatever, just to be clear i'm trying to rotate it by hand, and it slips slightly on its spindle, but doesn't actually move appreciably like one would expect a mechanical advance.

this is a really badly lit video that tries to show the timing (but i think it's impossible to see) and then let you hear the sound of opening the throttle, all with the grounded jumper that takes the computer timing offline. unfortunately the sound has static, but what you would see and hear is: a) fairly steady timing at ~7 deg BTDC, and b) it really struggling to rev up in RPM, actually dropping when i open full throttle quickly, but going up if i feather it.
https://photos.app.goo.gl/b6YuwfJ37TyFvGQYA

and this last is a video of the same as above with the jumper disconnected, ie. computer controlling advance. in this case, the revvs are better and can get pretty high, but still not as responsive as would expect. also, the timing at idle jumps around every revolution w/in about 5 degrees. when revving high the advance is strange, sometimes it goes way advanced as you'd expect, then drops back to just 15 deg or something, then way up high again. not sure if that's expected behavior or not.
https://photos.app.goo.gl/r6kZceUD7vTPhV2d9

thoughts?

[xboxrox]

Different distributor than on my 1986 Ram50 ~ hm, dark vid & photos I see rusty parts ~ clean clean or order rebuilt ?

[camoit]

I’m. So it’s running but when you hit the throttle it is very slow to respond and back fires, YES?
if that’s that’s the case it’s probably off one tooth on the cam timing. Many times someone will put the belt on and use the flat of the head as the mark or the belt is getting ready to snap. The timing mark is actually below the flat of the head on the passenger side. It’s a bump on the head. It the marks on the crank and cam are lined up and it’s still having problems it might be the MCA jet valves. You can check them with a vacuum gauge. When they go bad the needle is a whip.
If the timing is off 1 tooth the engine will run but but have no power. In the tech section we have pictures of the mark.
also a bad coil will keep it from revving up and can cause erratic timing