Misfire under load when accelerating

[gators_2006]

Hello everyone. New to the forum, and come seeking some advice.

 

I have tried to search for my issue, which it seems a lot have, but every thing I come up with, is for a year or two newer model than what I got. So, let me get to it......

 

Got a 1986 MJ (of course), with the 2.5L, 4 speed manual, 4x4, blah blah blah...... Love the little truck. Especially the fact that it still has AMC branding on many of the parts. Heck the title shows it as an AMC truck.

 

The issue I am having, and it seemed to come out of nowhere the last few days, is when accelerating, it misfires and bogs down, but only through a certain RPM range. I don't have a tech, so I can't tell you what rpm's it's missing at, but it's probably a range of 500-600 rpm's starting at 2000 rpm's would be my guess. Anything before that and it's nice and smooth. Once I get past that range, it's fine. I can push in the clutch and rev it and it never has the first miss or anything else, rev's just fine. This only happens when in gear, and it doesn't matter what gear.

 

It will do it when the weather is cold, it will do it when it's hot, doesn't matter. Cold start or been running for hours, still does it. Every once in a while it will act like it clears up by itself, but not often. Happens on hard acceleration or accelerating like grandpa.

 

Any ideas? 

[jdog]

Check your motor mounts first.

Next question is when was the last time a tune up was done?

Welcome, others will chime in as well

@eaglescout526

[eaglescout526]

What’s the history on the 2.5L? Anything been changed? Cruisers tips? Sensors adjusted? I only ask cause there’s some stuff that can make your engine do that and it sounds similar to when I had a bad head gasket and it would hesitate to accelerate. I also have the TPS go bad and while driving it would deliberately decelerate and then pick back up like nothing happened. 

[89 MJ]

I would check sensor adjustment and check all of the vacuum lines on the engine and go from there.

[87MJTIM]

Fuel delivery issue?  It bogs down at higher RPMs while driving but not parked.  Could the engine be starved under load?

 

How old is the fuel filter?  TB injector partially clogged?

[gators_2006]

History of the truck.

 

I bought it end of last spring. It had a leaking brake booster, so I disconnected the vacuum line and capped the line (who needs power brakes on a small truck!). That resolved the extreme high idle. I also replaced a few other vacuum lines that didn't look all that great. Other than that, I haven't done anything to it. Been running like a top, until a few days ago when this started happening. The guy I bought it from did put new plugs and wires on it, I'm not going to bad mouth anyone here, but after talking to him when I was buying it, I could tell he was throwing parts at it to fix the idle issue, not knowing it was vacuum related. 

 

I do want to pull the plugs and check to make sure they were gapped correctly and figured that while I have them out, might as well put my pressure tester on it and check compression on each cylinder. The block still has factory paint on it, but the head doesn't. Looks like some head work has been done previously.

 

That's about all I can tell you as far as engine history. Brakes were a different story............

 

 

[Ωhm]

3 minutes ago, gators_2006 said:

The guy I bought it from did put new plugs and wires on it,

Check distributor cap, look for crossfiring.

[gators_2006]

5 minutes ago, 87MJTIM said:

Fuel delivery issue?  It bogs down at higher RPMs while driving but not parked.  Could the engine be starved under load?

 

How old is the fuel filter?  TB injector partially clogged?

I'm going to check the filter this afternoon. I know it looks new, but that doesn't mean anything. I have run a few cans of seafoam through it since the guy I got it from was using it as his "dump" truck once a month or so when he needed to make a dump run. But even at that, I've probably run 100+ gallons of fuel through it in almost the year that I've had it. The TB does look like it a little dirty, but they all tend to do that over time. I want to take it off and give it a good cleaning, but I want to fix a problem first, before I start doing "side" projects that might make the first problem worse and compound what's going on. 

[gators_2006]

5 minutes ago, Ωhm said:

Check distributor cap, look for crossfiring.

Will do. 

 

I thought about the cap and the coil, just as preventative measures, but then I forgot how old I'm getting and that I should be responsible, and for a minute or two was contemplating what would happen if I just so happened to accidentally dropped in my MSD control box and setup I use to use when I was roundy round racing and running the strip. I mean it was all on a v8, so with 4 cylinders, it should be great! But then I came back down to earth and threw that idea out the window, and never got back around to checking the cap.  

[Jesse J]

3 hours ago, gators_2006 said:

. Love the little truck. Especially the fact that it still has AMC branding on many of the parts. Heck the title shows it as an AMC truck.

that's why I love mine too. amc for the win. like the seatbelts and the logo in the cluster

[eaglescout526]

I would start doing some sensor tests too just to make sure they are being read right by the ECU and do the grounds as well in cruisers tips. I would also check the coolant just to be on the safe side. Also if he put on a new TPS, it may not be adjusted properly. 

[gators_2006]

All sensors look like they have never been messed with, at least not in a long, long time. I didn't check compression yet, but I did pull the plugs and they look good, all 4 look the same. Cap was clean, no build up and rotor was clean. Pulled the fuel filter and the back flow from it was clean, but almost looked like milk coming through it. So I changed the filter. 

 

Still no change. I did notice that, if I run through 1st and 2nd pretty hard, I notice a slight bog, but it pushes through it with no problem. If I ease into 1st and 2nd, it's real noticeable.  Once I hit 3rd and 4th, due to the gearing and that it takes longer to run through those gears, I really notice the bog and the miss. More so if I am trying to go full throttle vs babying it. But either way, it's there. But, as usual, not a hiccup when in neutral.

 

If start testing sensors, would the TPS be the first one everyone would check first? Plus, where would I need to look for grounding in the cab for the instrument panel, etc...... The ground is good from the battery to the block, but I'm going to swap out a few other of the grounds and beef them up along the way. I work for a utility company so I get #6 flat braided ground wire by the 25 foot rolls! 

[Ωhm]

When LOGGED-IN. Go to upper right hand corner of screen. Click on Username. Click on Account Settings. Find and click on Signature. Add your vehicle information here (YR, ENG, TRANS, etc, etc) then click on SAVE. This helps everyone. This information will be available every time you post.

[eaglescout526]

Milk like oil? Hmmmmm, something tells me like what you are experiencing what sounds almost exactly like when I had a blown head gasket. It first started with the constant brown coolant, then later on while driving, she would bog and then clear right up and sometimes when driving at lower speeds, she would bog and sputter like no tomorrow until I would give her full throttle. Honestly, I would look into checking the head gasket if the engine has never been serviced. 30 years is a long time for the original head gasket and they do not do well when coming back to life either. Plus it would give you an idea as to what the internals are like regardless if the gasket is good or bad.  

[gators_2006]

10 hours ago, eaglescout526 said:

Milk like oil? Hmmmmm, something tells me like what you are experiencing what sounds almost exactly like when I had a blown head gasket. It first started with the constant brown coolant, then later on while driving, she would bog and then clear right up and sometimes when driving at lower speeds, she would bog and sputter like no tomorrow until I would give her full throttle. Honestly, I would look into checking the head gasket if the engine has never been serviced. 30 years is a long time for the original head gasket and they do not do well when coming back to life either. Plus it would give you an idea as to what the internals are like regardless if the gasket is good or bad.  

Sorry, I'll try and edit that one. My mistake. I was talking about the inline fuel filter on the frame. The fuel that was in it, when dumped out was clear, coming from the "IN" port, but was so cloudy coming out of the "out" port. So I know the fuel filter isn't an issue now that I changed it.

 

I am going to do a compression test hopefully by this weekend. My tester is out at our farm and I just haven't had the chance to go pick it up yet. I di throw a small tach on it last night as well.

 

This one has me stumped somewhat. I did notice this morning that it really isn't at a certain rpm range like I originally thought. I could be at 1800, or I could be at 2800. Didn't matter. It would be nice and smooth (once it quit bogging and missing) and as soon as I press the pedal about 1/4 of the way, it starts bogging and missing. Once I get past about 1/2 to 3/4 pedal, and the rpm's come up, it stops acting up. So that has me wondering if it might be the TPS. But I could be wrong.... won't be the first time and won't be the last.

[eaglescout526]

Ah, makes sense. Compression test ought to still help show the health of the engine. 
As far as the TPS, test it and see what it’s readings are with the key on engine off. You want to make sure it’s getting 5 volts to the sensor and the TBI manual says output and wide open throttle should be around 4.64(could be wrong, have to look) volts. 

[gators_2006]

I checked the TPS tonight. I put my meter on B and C and got my 5vdc. When going from B to A, I had .348vdc at full close, and as I slowly opened the throttle, it adjusted to 1.01vdc at around half throttle ad then dropped to 0.001vdc. I'm not a mechanic by trade, but I wouldn't expect the voltage to drop to O at half throttle. It should level out and stay constant, correct? 

[cruiser54]

Back to basics.

 

Every 30,000 miles, they should have new plugs, wires, cap and rotor. 

 

Notorious for intake manifold bolts loosening up.

 

Go to www.cruiser54.com and complete Tips 1 through 5 before running down rabbit holes.

 

I've seen driveability issues caused by the bad ground on the firewall. 

[gators_2006]

55 minutes ago, cruiser54 said:

Back to basics.

 

Every 30,000 miles, they should have new plugs, wires, cap and rotor. 

 

Notorious for intake manifold bolts loosening up.

 

Go to www.cruiser54.com and complete Tips 1 through 5 before running down rabbit holes.

 

I've seen driveability issues caused by the bad ground on the firewall. 

Thanks for the site info. I'll definitely be reading through that. It's been a while since I have worked on these things. Wife always has a new(er) SUV that you have to be a MIT grad to work on, and I drive an old '94 F250 7.3idi. 

 

It's been a while since I have toyed with one of these tings. I bought a bone stock, RV towed everywhere, '94 XJ in the mid 2000's. I knew that thing inside and out, and went from bone stock to a pretty decent trail ride. After starting to have kids and realizing it was more important to put money towards their food, and not the XJ's crank sensor (I kept spares in the glovebox), I let her go. I swear I was running behind that thing in slow motion, crying, reaching for it as a toddler reaches for his mother, as it pulled out the drive. I think I even heard depressing music playing from out of no where as that unfolded......

 

So needless to say, I have been waiting to get my hands on another. Wife goes on a weekends girls trip, tells me how much she loves the new Gladiators and that she wanted me to get a "Jeep Truck". So I did...... just not the Jeep truck SHE was wanting.

 

But I digress from the main topic.

 

I did notice last night, after I as checking the voltage on the TPS, and moving it to wide open several times, it didn't seem to act up as much on my way home from work. When I got home, I looked closer at the TPS and noticed that unlike the others that I'm use to seeing, this one is not mounted flush with the TB. It' off by 1/2"-3/4" and it has a metal arm that rests on a TB link. Don't know it this was a poor design, or what. There was a lot of dirt and grease almost covering the entire "linkage" between the TPS and the TB. I took some cleaner I had and really sprayed it all down and got all up in their and made sure I cleaned around the mechanism where it went into the TPS housing. Haven't checked my voltage this morning, but it never had the first hiccup on the way to work. Makes me wonder if there is enough of a gap between the TPS housing/body/whatever you want to call it, and the mechanism /linkage that comes out and goes to the TB link.

 

So with the age of everything, and it seeming like the TPS is at least part of the problem (voltage dropping to 0 at half throttle when testing before cleaning), I will go on and order a new one, and proceed with giving everything else a thorough "once over".

 

Thanks for the suggestions and info everyone.

[eaglescout526]

TPS voltage signal will make it act funny as it tells the ECU how much fuel it needs to dump into the engine to keep up with the acceleration and what not. The TPS is supposed to be flush with the TB and the linkage shouldn’t have any obstruction to the TPS arm. So you could’ve found your problem. But just keep an eye on it for a bit and see if it acts up. 

[gators_2006]

4 hours ago, eaglescout526 said:

TPS voltage signal will make it act funny as it tells the ECU how much fuel it needs to dump into the engine to keep up with the acceleration and what not. The TPS is supposed to be flush with the TB and the linkage shouldn’t have any obstruction to the TPS arm. So you could’ve found your problem. But just keep an eye on it for a bit and see if it acts up. 

That's what I'm use to seeing, a flush mount TPS. But this one was designed like that. I included a stock pic of the type I have. Except mine is has a small river holding the "arm" to my TPS and not a screw.

 

[eaglescout526]

Right, I have a 2.5L, trying to help. That is the TPS that mounts to the back of the TB and it should be flush. You are right about the arm should just sit right on top of the TPS arm and actuate. The front is the ISA motor and that’s a different animal on its own. I’ll break out my TBI manual this evening to help provide info on the TPS. 

[cruiser54]

If the grounds are not good, the TPS reading, along with other sensors' readings, will be skewed. 

 

CRUISER'S MOSTLY RENIX TIPS

RENIX GROUND REFRESHING

OCTOBER 30, 2015 SALAD 60 COMMENTS EDIT

 

The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, failed emission tests, and wasted money replacing components unnecessarily.

All the components listed below ground at the dipstick tube stud:

Distributor Sync Sensor, TCU main ground, TCU “Shift Point Logic”, Ignition Control Module, Fuel Injectors, ECU main ground (which other engine sensors ground through, including the Oxygen sensor, Knock Sensor, Cruise Control and Transmission Sync signal. All extremely important stuff.

The factory was aware of the issues with this ground point and addressed it by suggesting the following:

• Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. If the whole stud turns, you can use a 7/32″ six point socket or wrench to hold it so the nut can be removed. Worst case, cut the wires and remove the stud and nut. Install new terminal eyelets on the wires when going back together.  Scrape any and all paint from the stud’s mounting surface where the wires will attach. Surfaces must be clean, shiny and free of any oil, grease, or paint.
• Inspect the wire terminals. Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Apply a liberal coating of OxGard, which is available at Lowe’s and other stores. Reinstall all the wires to the stud and tighten the nut down securely.

While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, apply OxGard, and reattach securely.

Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for its intended use and subject to corrosion and poor connections at each end.

• Remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Apply OxGard. Reattach securely.
• Remove the other end of the cable from the rear of the head using a 3’4″ socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely with a liberal coating of OxGard.

2 STRONG suggestions regarding the ground system:

I prefer to add a #4 gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18″ long with a 3/8″ lug on each end works great and you can get one at any parts store already made up. NAPA has them as part number 781116.

A further improvement to the grounding system can be made using a #4 cable, about 10″ long with 3/8″ terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. NAPA part number 781115.

For those of us with Comanches, it’s very important to remove the driver’s side tail lamp assembly to access the ground for the fuel pump. Remove the screw holding the black ground wire. Scrape the paint from the body and corrosion from the wire terminal.  Add a 10 gauge wire, with an eyelet on each end, from that grounding point to a bolt on the frame. Better yet, on both Cherokees and Comanches, complete Tip 29 for the best fuel pump grounding. Be sure to scrape all mounting points to bare metal and apply OxGard also.

If you want to UPGRADE YOUR GROUND AND BATTERY CABLES with custom made parts, contact Paul at www.jeepcables.com 

Revised 02/04/2017

[LowerEnder]

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