*Solved* L400 S2 diesel- Stumble/hesitation around 2000 rpm

GC8Steve

GC8Steve

New Member
*This has been resolved. The issue was the alternator. Specifically diode failure creating an issue for the electronic injection pump ECU.*

Hey all.

I've put some miles (kms?) on my new to me 97 L400 S2 now.

I've been having an issue with a surging/stumble/hesitation that occurs around 2k rpm. Today I finally was able to replicate the stumble ever so slightly in park revving it up to around 2k where it presented as a mild vibration.

Hoping you guys can help me sort out the best way to go about troubleshooting. It's tough because 85% of the time it's not terrible. 10% of the time it's non existent, and maybe 5% of the time it's quite significant.

I typically notice the issue at mild load and lower RPMs close to 2k. Cruising speeds around town typically. As it begins to stumble, I give it some throttle and as soon as the torque converter unlocks and it revs up above 2200 or so it's smooths right out.

On the rare occasions it starts stumbling quite badly, I notice slight puffs of smoke coming out of the tailpipe.

Even when the stumble is significant, shaking the whole van, as soon as I give it more throttle it's smooths right out until I get back into the lower revs and mild load.

I've done the fuel filter, air filter, all the fluids and normal maintenance items. I just started a tank with some Diesel Kleen, though from what I've gathered here, since it starts without trouble and idles without trouble, it's likely not to do much for me.

The condition does not seem to be related to engine temperature. Sometimes it runs like this right after starting cold, other times I can drive for an hour with barely any issue and as soon as I restart with the heat soaked motor after a quick stop it behaves poorly.

It hasn't affected drivability much, because I can just lay into the throttle a bit and the problem goes away. But it's definitely annoying at cruising speeds in that rpm range where it stutters. And of course I'm concerned about doing lasting damage.

I'm thinking a sensor maybe? But I really don't know where to start and I'm new to diesels. It feels like a misfire, I wish I could just swap out some plug wires and fix it lol.

Any advice would be really appreciated. Thank you!
 
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Flat spot in the throttle positioning sensor? Put an analog meter on it and test for signal and observe
 
S2 should have the electronic controlled injection pump. This one can be a bit more of a struggle to figure out.

Has it run like this since you got it? Or did it start doing it after a service? (Filter change, new battery....etc)

Clean the battery terminals if you haven't and make sure the battery is good.

The fuel supply is sucked by the injection pump. So from the tank to the injection pump is under vacuum. If we draw air into the system it can make the engine stumble and or surge and or smoke. One of the things that can be checked is the fuel return coming out of the pump. If air is getting into the pump, air will be coming out of the pump. A temporary clear line on the return hose will tell us.
 

JOBD diagnostics; error codes

Copied from delicaclub.ca: 1994+ Mitsubishi Diagnostic Trouble Code Retrieval Without Scan Tool Ground Data Link Connector terminal # 1. Pin4 and pin5 are ground so you can connect pin 1 and pin 4 with a paperclip or short wire (or use another ground point if preferred). Set the ignition...
delicaforum.com delicaforum.com

Here is the way to get Trouble Codes from your van. I don't think you have a TPS on yours. Yours will have APS probably on the firewall inside the vehicle.
 
@DustAngel I think I have a line on a scope I might borrow.

It's also been suggested to me, and I recall reading a thread a bit ago, that the alternator could be failing and sending some stray AC through the system. And the scope would certainly help me verify that.

@Scott thanks for the link to reading the codes. It has run like this since I got it. Though I hardly drove it (like just around the block) before doing the initial service where I did the fuel filter.

It does have two brand new batteries in it, put in at yard before shipping. And the terminals seem to be clean.

Interesting trick on the temp return hose to view for aeration in the fuel.

Also, hi neighbor. Im over in Sterling. I see you are in the DC area as well. I'd love to link up with some local Delica owners one of these days.
 
So update on this. I'm leaning towards alternator failure. I just went out to test it. I put a multimeter on and didn't pick up any AC.

However, when holding the revs right at the stumble (hand throttle is great for this!), I was able to make it worse/better when introducing additional electrical load.

I'll update once I have a new alternator installed.
 
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