Author Topic: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load  (Read 3385 times)

JohnnyBfromPeoria

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Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« on: September 26, 2017, 08:05:12 PM »
Just like the title says, my 95 SR with something like 190K miles on it starts running really rough under load, especially at moderate-to-light throttle when hot, and when I mean hot, I don't mean after warmed up, like it switches into open-loop ECU operation. It takes several miles, well after my temp gauge reaches normal for this to start happening. It feels like it's dropping two cylinders, which at least from my seat on the couch, sounds like a bad coil. If I floor it, it runs o.k., if I back off, it bucks. At idle, it seems fine.

Just another day in the life of a 22 year old truck. Any random thoughts? P.S., to this point, I've done literally nothing to actually troubleshoot it. No check engine light comes on, either. It does have aftermarket coils in it that I replaced (the perfectly good stock ones for no good reason) a few years ago. Plugs are Iridium and the plug wires were replaced at the same time.

John B.
AZ Crew/East Sider/Former 14th St Crew
95 SR, 2 sets of tires rotting in the sun, 2" BL, stuff cut off, stuff welded on, lights, sound, no action
87 Raider, There's a turbo 2.6 under its hood, really
83 "Dodge" Power Ram 50, a bit lifted, way slow and gets more comments from random people than the other two put together
'95 Montero SR. Pretty much stock, Trail Gear Sliders, ADD skid plates, Stereo by Shovel, Timing maintenance by Pa_Jero

Shovel

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #1 on: September 26, 2017, 11:55:23 PM »
I don't want to muddy the waters but I'd be looking at MAF pretty early in troubleshooting.   Got a spare you can swap in? 

JohnnyBfromPeoria

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #2 on: September 27, 2017, 05:14:52 AM »
I do not but I'll bet the one from Kevin's 95 here in my yard would work. He took that Saturday though.

John B.
AZ Crew/East Sider/Former 14th St Crew
95 SR, 2 sets of tires rotting in the sun, 2" BL, stuff cut off, stuff welded on, lights, sound, no action
87 Raider, There's a turbo 2.6 under its hood, really
83 "Dodge" Power Ram 50, a bit lifted, way slow and gets more comments from random people than the other two put together
'95 Montero SR. Pretty much stock, Trail Gear Sliders, ADD skid plates, Stereo by Shovel, Timing maintenance by Pa_Jero

PA_JERO

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #3 on: September 27, 2017, 04:09:22 PM »
If you need a MAF to try you can use that one. I mostly just needed the hose. Let me know if I can help.

One thing that it could be is the coil boots. If the boot on the coil pack deteriorated at all or cracked, it causes that same effect. Just like yours, once it gets to operating temps and the engine is hot, happens just like that. We experienced this in one of adams trucks forever ago. May need to pull the plenum and put some known good coil packs on.

JohnnyBfromPeoria

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #4 on: September 27, 2017, 05:15:39 PM »
Just got home and my new coil packs, spark plug wires and plugs are here, so I'll start to getting the plenum off in a few. I have my spare plenum on the table in front of me to swap over the bushings and linkage I need to complete my previously overhauled one, too. If this doesn't clear it up, and I should know by the weekend sometime, I'll try to get with Pa_Jero to attempt some other parts.

John B.
AZ Crew/East Sider/Former 14th St Crew
95 SR, 2 sets of tires rotting in the sun, 2" BL, stuff cut off, stuff welded on, lights, sound, no action
87 Raider, There's a turbo 2.6 under its hood, really
83 "Dodge" Power Ram 50, a bit lifted, way slow and gets more comments from random people than the other two put together
'95 Montero SR. Pretty much stock, Trail Gear Sliders, ADD skid plates, Stereo by Shovel, Timing maintenance by Pa_Jero

Shovel

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #5 on: September 27, 2017, 05:45:27 PM »
If they use the same maf as a 2000 sohc I have a spare you can use for troubleshooting

IncorpoRatedX

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #6 on: September 27, 2017, 06:07:29 PM »
coil pack ignitor maybe, driggs just dealt with this one, or one kinda like it on the MOD trip, that igniter gets hot and fails. i want to relocate mine off the engine as heat is what kills those little buggers.

PA_JERO

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #7 on: September 27, 2017, 06:50:12 PM »
I have a spare one of those igniter dealio's. We can try that if the problem continues. I just replaced mine during my engine work.

Shovel

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #8 on: September 27, 2017, 07:27:08 PM »

IncorpoRatedX

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #9 on: September 27, 2017, 08:02:46 PM »
im not as electrically charged as you, but from what I see; that's a cap to reduce noise in the electrical system from the pulse of the ignition coil maybe? Pretty sure i've seen the cars and trucks run without that little bugger. Do you hold more knowledge on that little black box?

Shovel

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #10 on: September 27, 2017, 09:13:25 PM »
that's a cap to reduce noise in the electrical system from the pulse of the ignition coil maybe? Pretty sure i've seen the cars and trucks run without that little bugger. Do you hold more knowledge on that little black box?

It's a cap, and it's there to reduce noise.   Since we usually associate noise with sound the most common assumption is that it's there to prevent noise in the speakers through the radio.    But that's not it,  the power supply in the radio has its own filters, this thing is part of the engine management system and would be there without a radio.   

The cap that's connected to the ignition coil ignitor is there to limit the amount of electrical noise present in all of the engine management system (the ECU and all the sensors drink from the same straw..) and the coil ignitor & coils are the rowdiest assholes in the circuit.     

In that sense it serves kind of like how a torque converter in an automatic transmission eliminates jerky starts like when a novice is driving manual - which is more comfortable.     But the cap is much too small to be there for the purpose of absorbing that draw & shielding the rest of the electrical system for it,  waste spark coils are going to eat almost 20A when they fire.     The only reason a capacitor that small would be implemented is to drain out other noise,  apart from the expected ripples when you simply switch on and off a ~15-20A instantaneous load.     It's also not sized to concern itself with frequencies as low as engine RPM, it's an open circuit at those frequencies. 

The guy on that forum is speculating that the other noise could be from back-EMF - the electrical equivalent of your engine getting spun up by engine braking down a hill - because ignition coils go through a noisy stabilization phase while building up their magnetic field (known as dwell) and in some systems that can be a significant amount of voltage getting shoved into the "exit" of a transistor - if you look at that circuit diagram the first place a spike like that would go is into the base terminal for the engine speed bus,  as in the same one that the crank position sensor cares about.     

If that capacitor's job is to drain off transient high voltage from the noisy dwell phase of the coils (it is positioned in the circuit to be effective in that role) and then it is removed from the circuit or fails due to age/whatever,  the next place that energy is going to get dumped is into the same part of the coil ignitor transistor that the ECU cares about for timing.     If it's robust enough to handle that without burning out, it may still introduce enough noise to run badly.   If it's not robust enough then the weakest link burns out, either coils or the ignitor or the ECU or the crank position sensor, they're all in the path.     

So here's my angle:  It might not be a problem.   It might be a problem.    The cap is not there for radio noise it's there for the ignition system to work correctly and since the vehicle is 20+ years old and we've all seen our share of forgotten connectors, forgotten ground straps,  and "fuck it this thing is just there for the radio to sound purty you don't need it!" attitudes there's a possibility the thing is simply not connected or not present.     If it was my truck I'd at least include that in my inspection and if there wasn't one,  I'd make sure to get one since the things it's there to protect are the ECU, ignition module and crank sensor all of which are more expensive than $12.   The part number from Mitsubishi is MB363405

« Last Edit: September 27, 2017, 09:16:08 PM by Shovel »

JohnnyBfromPeoria

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #11 on: September 28, 2017, 04:41:07 AM »
Nice find, Shoveler. I shall investigate! I appreciate the eyes on this issue. Before darkness thwarted my ham-handed mechanical activities last night, I made it to where I had to consult my spare plenum section to find out why my upper plenum would not come off the engine. There's one more bolt at the back of the row o' bolts I can't see. I'll get 'er done next time, hopefully I get home from work before nightfall today.

It's encouraging to hear (now two) people also lean toward a coil pack as a first investigative lead, since I have three new ones in my kitchen. At the least, I'll have new Iridium plugs and silicone wires (replaced one at a time so as to not hook them up wrong, amiright, Josh?) and possibly have found a bad coil pack. Man, I hope so.

I was thinking I may even have to go so far as to use my oven (I don't actually use it for cooking, so why not) to heat up each old coil pack before I test it with my super-good multimeter, since this issue doesn't present itself when the engine is cold. After all, resistance builds with heat. Should be interesting and make for some good pictures.

I bought new oxygen sensors (Bosch) for this beast a long time ago, but couldn't figure out how to get the friggin' electrical connectors undone. They came loose from the exhaust pretty easily, but I tightened them back in after I failed to get the wires loose. Anybody want to lend some fingers to that this weekend? I will buy the malted beverage of your choice. Even if it's root beer.

John B.
AZ Crew/East Sider/Former 14th St Crew
95 SR, 2 sets of tires rotting in the sun, 2" BL, stuff cut off, stuff welded on, lights, sound, no action
87 Raider, There's a turbo 2.6 under its hood, really
83 "Dodge" Power Ram 50, a bit lifted, way slow and gets more comments from random people than the other two put together
'95 Montero SR. Pretty much stock, Trail Gear Sliders, ADD skid plates, Stereo by Shovel, Timing maintenance by Pa_Jero

TOASTY

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #12 on: September 28, 2017, 09:28:09 AM »
When that manifold didn't come right off why didn't you hit it with a dead blow hammer?

JohnnyBfromPeoria

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #13 on: September 28, 2017, 06:21:31 PM »
When that manifold didn't come right off why didn't you hit it with a dead blow hammer?

Because I knew there was still a bolt holding it on. I got smarter (well, a little) with advancing age. It's off now.

John B.
AZ Crew/East Sider/Former 14th St Crew
95 SR, 2 sets of tires rotting in the sun, 2" BL, stuff cut off, stuff welded on, lights, sound, no action
87 Raider, There's a turbo 2.6 under its hood, really
83 "Dodge" Power Ram 50, a bit lifted, way slow and gets more comments from random people than the other two put together
'95 Montero SR. Pretty much stock, Trail Gear Sliders, ADD skid plates, Stereo by Shovel, Timing maintenance by Pa_Jero

PA_JERO

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Re: Rough Running 95 SR (3.5 DOHC) When Hot/Under Load
« Reply #14 on: September 28, 2017, 10:39:09 PM »
When that manifold didn't come right off why didn't you hit it with a dead blow hammer?

 ;D

I knew that was coming. Eric has a solid point, the only thing is the 3.5l DOHC engines are not equipped with those. At least my 94 isnt, and I believe 95 does not either. 96 is possible, since there was a few additional items added with the obd2 change over. Johnny, there is a test procedure on those coil packs to ohm them out, I believe they have to be within a certain tolerance, in case you didn't want to put them in your Easy Bake Oven. I will be out of town this weekend but return sunday afternoon sometime. If you don't get any assistance through the weekend or need a hand buttoning up some stuff, I can come help monday.