Random Dieseling

[TeKdo]

Hello Everyone.

I have a 1986 Comanche X with a 2.5 and an AX5 that I daily drive. I've done Cruiser's tips 1 thru 8 as well as fixing any vacuum leaks that I come across. The only "mods" I've done are an H4 conversion and added fog lights (both of which are on their own isolated circuit from the factory wirin   straight to battery, own custom fuse and relay block.)

 

Recently it has started to diesel when i turn it off intermittently. Each time this has happened it stops and dies after a second or two and floods the engine. I was wondering if anyone else has had this happen and if I'm on the right path for repair. I'm suspecting that the starter relay is starting to fail and stick open sometimes letting the alternator keep supplying power after the key is off.

 

I've been thinking about taking the whole charging system off of that circuit to replace it with a megafuse to have the lasternator go straight to the battery and then have the batter feed by itself to the starter relay. Should I do this or just replace the starter relay?

[eaglescout526]

Flooding? Like fuel is still getting into the engine? I would suspect the injector could have failed O-rings if fuel is still getting into the engine. 
 

Id have to look at my 86 electrical manual, but I don’t think the alt is a worry here. The key being switched off is what turns everything off. You could have a sticking or melted ignition switch or connector down the column causing this issue. 
 

The ECU controls the injector pulses and ICM. The CPS tells the ECU when to send spark. So if you turn it off and the ECU is still getting power, it’s gonna get the signal from the CPS to spark and it’s still pulsing the injector. Would thus lead to your engine still operating a bit longer after shut down. 
 

Id definitely take a look at the ignition switch. That’s a free diag without spending money on parts that are very well still in working order. 

[cruiser54]

Your intake manifold bolts aren't loose? 

 

[TeKdo]

4 hours ago, cruiser54 said:

Your intake manifold bolts aren't loose? 

 

Nope, I tightened them pretty recently, like 1 or 2 weeks ago. Sprayed some starting fluid near the flange to double check and make sure I had a good seal, part of my vacuum leak hunting i did.

[schardein]

Interesting fact- when an engine "diesels" after shutdown, it is actually running backwards.

 

Google is your friend here.  There are common things that cause an engine to run on after shut off, or "diesel".  Carbon build up in the combustion chamber and/or on top of the piston, incorrect idle speed, incorrect spark plugs, incorrect fuel (insufficient octane rating), incorrect ignition timing, or a combination of several of these things.

 

You might try running a tank of the highest octane rated fuel at your local gas station.  If that helps, it may indicate a need to try and clear carbon from the combustion chamber.  Again, google is your friend, running some Seafoam through the engine or misting some water into the throttle body at a medium rpm (don't hydrolock your engine!) are some commonly recommended solutions.

 

Of course, inspect the engine for any faulty or missing components, some engines had an anti-dieseling solenoid that forced the throttle closed at shut down.  Although I don't know if the 86 2.5L had that.

[eaglescout526]

11 minutes ago, schardein said:

Although I don't know if the 86 2.5L had that.

ISA opens up after shut down to be ready for the next start.

[TeKdo]

I'm pretty sure that it isn't ignition switch, every single ignition switched power source goes dead when the key is turned off.

 

Now that you mention it, i am a little suspect of the injector. But the spray pattern is good and it doesnt glug fuel ever.

 

The reason I'm suspect of the starter relay is because thats the only spot where there's power when i go to key off. Looking at the fsm, I think that the contact for the starter relay that gives power to parts of the truck sticks ocasionally and lets the alternator give power for a couple seconds to let the injector pulse a few times before unsticking and shutting off.

 

[eaglescout526]

Just because the interior goes dead doesn’t mean the engine is turning off. I’d still inspect the switch if it has never been done. It is known to melt as well. Not as frequent as the head lamp switch, but poor grounds would cause it to heat up. 

I really don’t think it’s the starter relay and only because it’s a $50+ part and would like to try to help find the actual problem, but if your gut says it’s the relay, then replace it. 
 

And if that turns out the issue, I’ll be damned and can say this condition can point to the starter relay. 

[gogmorgo]

Have you pulled the spark plugs to check for carbon buildup, or other signs of overfueling?

Dieseling is usually caused by very hot deposits igniting off air/fuel mix in the engine. It’s almost never caused by lingering power supply to the ignition.
Carbureted and TBI setups are more prone to it because there’s more volume of fuel in the intake after shut down than a port-injected vehicle. Carbs with their mechanical fuel delivery will simply keep going as long as the engine runs, but injected engines shut the fuel delivery off and don’t keep going beyond a few pops. 

Bad gas or excessive cold operation can also lead to carbon deposits, as can excessive oil burning in the cylinder.

[TeKdo]

Update: Dieseling is mostly gone. I did a check before the post of for if the connectors on the ignition switch were garb or starting to go out and nothing seemed wrong. Unplugged it and plugged it back in and its almost all gone. No longer diesels for a couple seconds, now it just diesels for a tick after going to key off, like one or two little combustion cycles after everything goes dead.

 

On 2/4/2024 at 12:46 PM, gogmorgo said:

Injected engines shut the fuel delivery off and don’t keep going beyond a few pops. 

Bad gas or excessive cold operation can also lead to carbon deposits, as can excessive oil burning in the cylinder.

Starting to think the little 2.5 is a little tired but i don't think that its something to lose sleep over.  Crankcase pressure seems a little high to me (when you take the oil filler cap off, there is a ton of air flowing out of the top). Not sure if that's normal for a Jeep, but with the little import cars I normally work on that's a little worrying. She doesn't burn oil...the intake tracks look clean looking thru the TBI, and the plugs and fuel system are fresh. I put new plugs, wires, coil, and fuel filter in it when I got the truck a couple months ago. The old fuel filter was super clean too, no dirty gas came out of it when i pulled it down and plugs looked clean minus one that looked super gross due to an old plug wire completely disintegrating.

 

On 2/4/2024 at 12:46 PM, gogmorgo said:

Bad gas or excessive cold operation can also lead to carbon deposits, as can excessive oil burning in the cylinder.

As for the bad gas...I'm not 100% sure if that could be a problem. The first tank that I ran through this truck was a can of Berrymans and about 10 gallons of California 91 Octane Shell V-Power. Now, I run the truck in Washington State on regular 87 Octane from Arco, same as my grandpa did for the 30 years he had the truck. It didn't really diesel up until recently after we had some cold weather (Not super cold, like in the mid 30s Fahrenheit) and a little bit of snow. I did a good bit of driving in that weather between shuffling around town for work and a couple snowy rescues (Maybe a max of 2 weeks operating in that weather.) With my other cars (Mid 90s FI Japanese Hondas and Mazdas) they didn't really have issues running for a week or two in this kind of weather outside of cold starts because their batteries are smol and kinda weak. Is the Renix system that different that it can't cope with that kind of operation or if I'm just using the truck wrong?

 

Other than that, Thanks everyone for the help!

[eaglescout526]

13 minutes ago, TeKdo said:

there is a ton of air flowing out of the top). Not sure if that's normal for a Jeep

Normal. 
 

14 minutes ago, TeKdo said:

Is the Renix system that different that it can't cope with that kind of operation or if I'm just using the truck wrong?

It can cope with the weather. Many with the 4.0 in the winter start just fine. 
have you cleaned all the grounds?

 

The only thing that should remain on after shut down should be the ECU extending the ISA for the next start up. 

[gogmorgo]

Yeah if +30°F is cold for you, you may be shocked to find out what temperatures my 2.5 has seen.

 

Cold conditions do tend to make the truck run richer while it’s warming up, which can lead to carbon buildup and dieseling. There’s a chance the thermostat is getting weak and opening early, which would lead to slow warmup times.

 

However, the one spark plug that looked super gross probably isn’t the only thing inside that cylinder that does. Have you had it out on the highway for a nice good long run? Get it good and warm, like an hour at 60+mph? It might take some time to burn off all the buildup in there.
 

[TeKdo]

2 hours ago, gogmorgo said:

Yeah if +30°F is cold for you, you may be shocked to find out what temperatures my 2.5 has seen.

That's what i meant by "cold" lol. I've seen a decent amount of guys on here in Canada and other colder states so I was wondering if something was just up with my Jeep because 30F doesn't seem cold at all. I just replaced the thermostat when the cold came and even blocked off half the rad with cardboard to try and keep it warm. I've been suspecting the coolant temp sensor though because when it's below 50 outside you gotta give it a bit of pedal for it to idle.

 

2 hours ago, gogmorgo said:

However, the one spark plug that looked super gross probably isn’t the only thing inside that cylinder that does. Have you had it out on the highway for a nice good long run? Get it good and warm, like an hour at 60+mph? It might take some time to burn off all the buildup in there.

 

I have had it at highway speeds for longer periods of time (before i blew up the old AX5...don't ask about it, it's a long story) and it did clean up pretty good. For the first couple weeks that I was driving it after getting it running (it had been sitting since 2006 and I've been driving it daily since the beginning of December '23) it was gutless. I expected it to be gutless because it was a 2.5 but man, having to downshift to 2nd to climb a mild hill was kinda infuriating. It did clear up and come back around (dunno if there was a sticky ring or just carbon buildup) but between the fresh ignition system, fresh charging system, updated grounds, and a tps adjustment along with all the snake oil I've thrown into it (Berrymans in the gas, Seafom in the oil before an oil change, and Marvel Mystery Oil) it came back around and has some guts now. Maybe it just needs more time to burn off the junk from sitting?

 

3 hours ago, eaglescout526 said:

Normal.

Wow, I wouldn't expect that amount of crankcase pressure to be normal lol

[eaglescout526]

44 minutes ago, TeKdo said:

Wow, I wouldn't expect that amount of crankcase pressure to be normal lol

Yeah it is. If you feel it really is above normal, check the vac lines and make sure they aren’t plugged like the one to the air cleaner and the one from the back of the cover to the base of the throttle body. 

[TeKdo]

5 hours ago, eaglescout526 said:

Yeah it is. If you feel it really is above normal, check the vac lines and make sure they aren’t plugged like the one to the air cleaner and the one from the back of the cover to the base of the throttle body. 

I'm not gonna sweat it, idle is normal and its not burning oil like you'd expect from wasted rings and all the vacuum accessories work as designed.