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Pirate_Staz

Truck bucks like a horse at 2500rpms for some reason

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I renamed the thread to better explain whats goin on, and the original title felt too long.

 

 

Bit of a long read but I'm stunped so here's everything that has led up to this post, also for clarification my truck is an '86 but the entire powertrain has been swapped with a '96 cherokee, computers and all:

 

This last labor day weekend I took a trip to Mexico, a sort of maiden voyage for my truck as its never gone further than an hour or so. It made it all the way there perfectly fine running in 5th gear 2300-2500rpms with a fully loaded bed of supplies and AC on no problem. However, as soon as I got gas at a pemex station in Mexico (which ill never do again) my truck started having issues. Id be going 65-70 and it would just bog down, lose power, misfire, and jerk back and forth. Even in nuetral if i attempted to rev it would be choppy and try to stall, and sometimes pressing the gas pedal would do nothing and the revs would stay at idle (about 650-700rpm). Temps read normal (even lower that 210 at some points) but i noticed my oil pressure would shoot up from 50-55 to 60-65. My alternator readings also shot up and would read in the middle when previously they read a little bit lower than middle. I would just end up hunting gears and downshifting to keep the revs as high as I could until it went away after about 5 minutes. This would continue to repeat for the rest of the trip in Mexico with it bogging for a solid few minutes before acting normal again for another 10 or so minutes. This really sucked as most of the time I was in the sand but my old girl pulled through every time and I ended up making it to the campsite without getting stuck. 500 miles from home in a different country there wasnt much I could do so i just hoped it was the fuel sending unit acting up and getting to hot and by the time we left to go back home itd be fine again. This was half correct as the day we left it ran normally until we got out of the desert and onto asphalt again, but after 10 minutes of asphalt driving my ac started smelling like something was burning, and straight up died and started blowing hot air. Immediately i turned it and the truck off to inspect under the hood and sure enough I'm definitely smelling some sort of burning smell, but I have no idea what. At this point I can also smell oil when it bogs down, but this is definitely something different that I still have been unable to locate.  I thought maybe its overheating and my thermostat is stuck and thus my gauge is reading incorrectly so i took the hood off and put it in the back, after letting it sit for a second i started it back up and continued on my way, checking the AC to make sure it blew cold again (it did) and then turning it off to make sure i wasnt creating needless heat. For the most part it did really good, i drove a solid 45 minutes at speeds of 65 with no issues but as soon as i smelled that burning smell and oil again it started bogging down. This time i said to hell with it and downshifted straight to 3rd at 55mph and 3500rpms. I drove like this for a good few minutes with no issues so i thought maybe it had gone away and shifted into 4th. Nope. Immediately bogging and misfiring and losing power, same for 5th, so back to 3rd it was. I continued in 3rd for awhile with no issues other than my engine sounding like a prop plane before checking again and this time it went into 4th and then 5th with no problem and that continued until i crossed the border. After the border I stopped at a Texaco and filled up 3/4 the tank with their standard unleaded that had techron and put some injector cleaner in as I suspected at this point maybe the gas I got in Mexico was a little watered down or just all around bad. After filling up I made it an entire 275 miles with 0 issues cruising at 70mph until i was about 10 minutes from home and it started bogging down again, only this time it didnt go away. I kicked that trucks butt in those final 10 minutes i tell ya and as bad as i felt about it I just wanted to get home, 2nd gear, 3rd gear, 4th gear, i was just shifting nonstop doing the best i could to keep it at least at 45mph even though it was fighting me the whole time. I finally made it home and the next day i filled up a new tank and added some more injectior cleaner and took it out on the highway where low and behold- nothing happened it drove perfectly. On the way back however after turning it off to get lunch and coming back it started acting up as i was leaving the parking lot and straight up stalled out on me. That burning smell was also present. 2 days since then now and it consistently does it on a cold start if I don't give it a few minutes to warm up and I'm at a total loss for what could have caused this whole ordeal. Clogged fuel filter? Bad gas? Bad alternator? Bad injectors? Bad fuel sending unit/fuel pressure regulator? Not getting a good spark? It also seems like my truck is down on power now too, or rather the pedal isnt as responsive. It accelerates much slower than it was even though now it doesnt even have all the weight in the back from the trip. Thanks for reading this far I'm just having trouble locating the issue as it seems it could be tons of different things. This truck is my daily so ill continue to drive it every day and it seems fine once it warms up but like I said I can't start driving after a cold start (or even a warm start) without giving it a second or else itll misfire/bog down and try to stall. Let me know what you guys think! here's a pic of my truck going "roadkill" as a reward for reading this far:

20190901_191443.jpg

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Have you changed the fuel filter?

There were some coil failures in the 90s on the HOs. see if yours is a Marelli. That's the one that failed.

 

Test your fuel pressure with a gauge. Is the fuel pump loud? Since it is a Comanche, I would for sure do the fuel pump ground upgrade just cuz. 

 

How old are cap, rotor, plugs and wires? Have you inspected them since you got home? 

 

Oil smell like gas? 

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If you added diesel inadvertenty, the results would be just about what you describe. Do you speak Spanish? The English word diesel in Spanish is diesel, gasoil or gasoleo. English gasoline is gasolina. (Tildes not included.) Still have your receipt? Maybe you put gasoil in your tank.

 

Did you get smoke from your exhaust also? 

 

If you did put diesel in the tank, run your tank as close to empty as you can for the next few tank fills and the problem should go away. After that you might want to check the fuel filter for trash you may have stirred up that was already in the tank because of running down to empty. 

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On 9/6/2019 at 8:47 PM, cruiser54 said:

Have you changed the fuel filter?

There were some coil failures in the 90s on the HOs. see if yours is a Marelli. That's the one that failed.

 

Test your fuel pressure with a gauge. Is the fuel pump loud? Since it is a Comanche, I would for sure do the fuel pump ground upgrade just cuz. 

 

How old are cap, rotor, plugs and wires? Have you inspected them since you got home? 

 

Oil smell like gas? 

Ive looked my truck up and down and can't figure it out. The electricalish burning smell I was smelling I believe is coming from my power steering pump, as the pump whines and hums pretty gosh darn loud even at idle and while turning now, but everything else looks like its in order.Ill check my coils to see who made them and if youre correct ill replace those. I'm planning on replacing my fuel sending unit/pump assembly anyway to be on the safe side, but other than stuttering/bogging a bit while getting up to speed and the occasional misfire in 3rd or 4th It really hasn't done anything as bad as it did in mexico. The oil smelling like gas is a good point though and Ill definitely check on that. How would gas be getting into the oil though? 

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On 9/6/2019 at 11:06 PM, Manche757 said:

If you added diesel inadvertenty, the results would be just about what you describe. Do you speak Spanish? The English word diesel in Spanish is diesel, gasoil or gasoleo. English gasoline is gasolina. (Tildes not included.) Still have your receipt? Maybe you put gasoil in your tank.

 

Did you get smoke from your exhaust also? 

 

If you did put diesel in the tank, run your tank as close to empty as you can for the next few tank fills and the problem should go away. After that you might want to check the fuel filter for trash you may have stirred up that was already in the tank because of running down to empty. 

That was the first thing I checked, but I'm 100% positive it wasnt diesel I filled the tank with. The 2 other jeeps I was with also used the same station and same pump and they had 0 issues. My dad was driving behind me the entire time and he said there was never any smoke. Ive already run the tank downa few times and also run some fuel stabilizer and injector cleaner through it but it still gives the occasional lurch while cruising at 65-70 and bogs occasionally in gear/getting up to speed. For the most part the issue has gone away though and I'm hoping its just my pump giving out and replacing it later this week will solve all the issues.

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If you are sure that you have not loaded diesel, then you may have something other than (or in addition to)  a fueling issue going on.  How soon after you filled at the PEMEX station, did the problem start?  Within minutes, or an hour or more later?  You describe having the problem when you have a load against the engine. How old is your cat converter and muffler?  If you were bouncing down dirt roads after leaving the PEMEX station, you might have shaken a rusted out cat converter and muffler so that the rust collasped and blocked exhaust flow.  If  that happened, that would account for the loss of power, the smells and miss fires.  FYI, Petrõleos Mexicanos, PEMEX, is government owned and an important source of revenue for the country. Quality is monitored. It is not likely they contaminated the output. 

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5 hours ago, Manche757 said:

If you are sure that you have not loaded diesel, then you may have something other than (or in addition to)  a fueling issue going on.  How soon after you filled at the PEMEX station, did the problem start?  Within minutes, or an hour or more later?  You describe having the problem when you have a load against the engine. How old is your cat converter and muffler?  If you were bouncing down dirt roads after leaving the PEMEX station, you might have shaken a rusted out cat converter and muffler so that the rust collasped and blocked exhaust flow.  If  that happened, that would account for the loss of power, the smells and miss fires.  FYI, Petrõleos Mexicanos, PEMEX, is government owned and an important source revenue for the country. Quality is monitored. It is not likely they contaminated the output. 

id say a good 20 minutes passed before I started having issues. My catalytic converter is 33 years old but my Muffler is brand new. Its possible tht some rust was shaken loose but I live in AZ and rust is pretty scarce here. Ill check the cat though just to see if theres any loose debris in it and maybe replace it altogether if need be thanks for the tip

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The heat of the cat causes the rust, that and the water that is a combustion product. Even if the cat isn't rusted out though, 33 years down the road it's probably not doing much for you anymore anyway. 

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On 9/10/2019 at 7:58 PM, gogmorgo said:

The heat of the cat causes the rust, that and the water that is a combustion product. Even if the cat isn't rusted out though, 33 years down the road it's probably not doing much for you anymore anyway. 

lol youre probably right, probably couldnt hurt to replace it

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Some new developments, today was a pretty humid day and the truck once again lost power on the highway, once while getting onto the highway in 3rd around 2500rpms it just stopped and even though I was pressing the gas it went down to idle, after shifting into 4th though it was fine, and at the end of the day coming home on the highway I was going up a hill losing speed in 5th so I downshifted to 4th and it immediately lost power and went to idle even though I was pressing the pedal. Shifted back up to 5th and it was fine (albeit very slow since I hadnt made it up the hill yet.) What a weird little quirk my truck is developiing. My dad thinks its electrical and the humidity is causing it, but what could the humidity be affecting? My oil pressure and alternator gauge readings also went up again like they did in Mexico while it happened. Also worth noting i guess that right before the first time it happened today my speedo was acting wonky and was jumping all over the place, as soon as the bogging stopped the speedo went back to normal. Maybe my truck is just haunted.

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Later this week Ill be replacing the Battery since mines from 2014, the alternator since I got it tested and they said the voltage regulator failed, and the fuel sending unit in the tank which has the regulator, pump, filter all attached to it. Hopefully this helps or else It looks like Ill be going through all my wiring and checking for shorts/broken wire insulation etc. Also probably going to replace the catalytic converter in the near future.

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Do the fuel pump ground mod!!!

 

You mention humidity playing a part. How old are the plug wires? 

 

Did you look at the coil as suggested?

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7 hours ago, cruiser54 said:

Do the fuel pump ground mod!!!

 

You mention humidity playing a part. How old are the plug wires? 

 

Did you look at the coil as suggested?

what is the fuel pump ground mod? plug wires look brand new, albeit theres some coolant stains on them from a much earlier problem thts now fixed. i checked that coil and it was just all black and i didn't see any brand names on it. It also looked as old as the engine so I'm assuming it won't kill me to replace that as well.

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popped off my muffle and took a look inside the catalytic converter, it didnt look broken up but when you shake the housing you can definitely hear something rattle around inside and the ceramic also moves freely as if its not being held in place. I don't think this is the cause of my issues but I guess it could be attributing to it? 

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Here ya go:

 

IMPROVING THE FUEL PUMP GROUND

The fuel pump and fuel tank sending unit ground at a sheet metal screw up behind the spare tire on an XJ, XJ-Body-Ground.jpg?resize=300%2C162and behind the driver’s taillamp on an MJ.

Not only is a sheet metal screw a lousy way to ground things, this ground path is long and travels through some connectors that are prone to corrosion and moisture.

Locate the black wire on  the HARNESS side of the fuel pump/sender 3 wire connector. Remove a 6″ length of the split loom covering. Strip back about 1/2″ of insulation from the BLACK wire.1-fuel pump ground MJ

Take your new ground wire, preferably at least 14 gauge and 12 to 18 inches long as needed, strip it about 3/4″, and wrap it around the exposed part of the harness plug wire.1-fuel pump ground mj-002Solder the connection. Tape it up and reinstall the split loom covering.1-fuel pump ground mj-003

At the other end of your new ground wire, add a crimp on eyelet. Attach the eyelet under a bolt that goes directly into the chassis. Be sure to clean the attaching point til shiny and apply OxGard to the contact surfaces.1-fuel pump ground mj-004

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more new developments, it only occurs as soon as I hit 2500rpms, if I stay under that while going up a hill/towing something its fine but as soon as I go over 2500rpms while under load it starts dying and hesitating on me

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1 hour ago, JustEmptyEveryPocket said:

Does your swap have a electronic control on the power steering pump? I'm thinking something along the lines of THIS VIDEO, finished in THIS VIDEO

That does seem super similar to the issue ive been having but i just checked and theres nothing electrical running off my power steering anywhere so I don't think its the same issue

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2 hours ago, Pirate_Staz said:

so I don't think its the same issue

I guess I was pointing more to the logical flow Eric had. Since you have something happening at 2500rpm exactly, what else happens at 2500rpm? E-fan kick on? IDK, but it seems like that should give you good info to start chasing something.

 

Could be a red herring as well. :dunno:

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I'm getting to the point where I can recreate it pretty consistently now. I know this entire thread has probably made me look like a mad man but I'm 90% sure once I get the new fuel sending unit from saveevryjp itll fix my problems. On a cold start if you accelerate somewhat hard you can feel it misfire and stumble, even more so if the AC is on. Once you pass 2500rpms itll cut out and buck without fail. It will occasionally do it in 1st or 2nd, but always will do it in 3rd or 4th. It goes away completely once the engine is at operating temp. Hopefully once I replace my fuel assembly thatll be the end of it, I know halloween is coming up soon but I'm tired of my truck acting like its haunted.

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Sorry for not updating for awhile but ive been busy. New fuel sending unit changed nothing except now the pump is quieter which is nice. my truck starting throwing codes finally saying my upstream o2 sensor isnt working properly (Code P0135 and P0132) so maybe thats where my problem lies. It still dies over 2500 rpms but at this point I'm tired of spending money on my truck and until I need to get emissions or something breaks I'm just going to ignore it. Thank goodness it goes away after my truck hits operating temps. A weird issue and one that I still havent solved 

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Alright alright alright Ive got another lead, I believe either A: my upstream O2 sensor is the incorrect one, or B: theres some sort of governor (possibly related to o2) that if my engine isnt at a specific temp itll put a rev limiter on my truck @2500rpm. If B is the answer that might explain why it happens only at cold start, or on the highway. My cooling system is just too efficient now yeesh, truck never sees 210 anymore its usually 180-195 range so that would explain why it starts happening on the highway. If anyone knows if these trucks/XJs in general (as mine is a 96 swap) have some sort of cold start rev governor let me know.

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Random suggestion, but check the voltage output of your alternator under load. If the voltage regulator in it is going bad and supplying incorrect voltage, weird things can happen.

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