'95 E300D Lumpy Idle
 

With the onset of cooler weather, my '95 E300D has begun to regularly show a very lumpy idle on startup. If I add the slightest pressure on the throttle, the engine smooths out. Typically, this hunting will go away as the engine warms up. However, on a few occasions, the engine starts with faster idle, around 1,000rpm, which then creeps up as high as 1,500rpm as the engine warms. I can usually eliminate it if I push the accelerator pedal down and let it snap back, like there's an issue with the linkage, though I've cleaned and lubricated the pivot points.

Two part question:
!. Is there a way to manually adjust the idle on the E300D? I don't see anything obvious that would allow me to do that, though I might be looking right at it. My prior experience with engines has been with gasoline-powered ones.
2. Is there some sort of sensor that controls the idle until the engine begins to warm up? If so, where would it be located?

Thanks for your thoughts and suggestions on this.

Regards,
Tom

Hi Barn guy the engine computer controls the idle speed based on engine temperature. I'm pretty sure the sensor is between the IP and block, that's the hard one to get to. If you had a 1k potentiometer you could simulate the different temperatures. I can't recall the resistance values but I keep a 1 k pot for such purposes. No adjustments for idle speed.

I think I would try a diesel purge first though.

I had a similar high idle problem on cold mornings. To me in Texas that is below 60.
Problem turned out to be the cruise control linkage rod to the mechanical control box lower right of injection pump. High idle would go away when engine warmed or if I raised the hood and pushed the top linkages back to normal idle. I tried spray lube on linkages and cables to now avail.

Linkage rod from cruise control to injection pump throttle causing high rpm til warm

Only time my idle was ' lumpy ' if that means a little shaky was after new motor mounts. That went away after a couple of months. Don't understand why but hasn't been a problem since.

I second the cruise control linkage. And while you're in there, make sure to lube all the ball and sockets and pivot joints.

Chris W.

'95 E300D Lumpy Idle
 

Thanks Texasgeezer and Chris W! Your experience mirrors mine as I thought the problem improved if I played with the linkage. Is it possible to get to the linkage from cruise control (which hasn't worked since I got the car) to control box on the injection pump without removing the intake manifold? It seems difficult to address from below without a lift.

Same experience for me on that cruise linkage. I would actually get stuck on cold mornings. First time it happened I thought I was about to have a runaway.

Don't know about accessibility, to me removing the intake manifold goes pretty quickly.

Just make sure to remove debris or broken injection pipe clip pieces that might fall into the intake ports before you loosen the intake manifold. I was lucky during 1st removal. I now use a shop vac and air blower to clean that area 1st.

Helps to use battery drill/light impact with magnetic bit holder to speed up removal of intake bolts.

I use colored zip ties to mark the vacuum lines and take photos to help put things back in place. If vacuum hose ends are cracked/swollen nip off a little bit to help keep a tight seal. Small zip ties can also tighten vacuum tube ends.

I also use a light coat of silicone grease and anti-sieze on appropriate places. Makes removal easier next time and reduces chance of damage to threads, electrical plugs, etc..

Intake manifold gasket has always been reusable, but I have spares if needed. I usually coat the new one with a light coat of oil/silicone to reduce chance of sticking during next removal.

Your lumpy startup maybe a bad glow plug or even a bad valve.

Usually a dead glow plug illuminates the glow lamp after the engine has started.

Zulfiqar: I doubt it's a glow plug since I changed out the glow plugs about two years ago, along with the engine wiring harness, resolving all the problems I had prior to that. As for a valve, very possible, but the lumpiness immediately goes away when the engine reaches a specific temperature. The lumpiness is there, then gone immediately. Also, if I raise the idle slightly with the accelerator--maybe 100rpm or less--I can make the engine run normally without any signs of hunting. Take my foot off the pedal when cold, and the hunting returns immediately.

'95 E300D Lumpy Idle
 

Yeah! I pulled the intake manifold off when I was doing the glow plugs and changing out the engine wiring harness. It was a relatively easy job.

I scoped out the engine today from underneath and there really isn't any way to easily access the linkage. Since the intake manifold needs to come off, I'm going to live with the idle issue for a bit. My cruise control hasn't worked since I got the car and I'd like to return it to an operating state and the manifold will need to be removed for that. I'm on the hunt now for a functional, used CC-unit. Once I get that and, if I understand correctly, change the brake light bulbs to ensure they are all OEM equivalent, I'll tackle both at the same time. I also need to change one last engine fuel line to complete a full replacement which I'll also do at that time.

Thanks for the manifold removal suggestions. There's a few there I'll work into my routine which I didn't use the last time.

Well the plenum has been pulled and all connections in the accelerator linkage have been disassembled, cleaned and lubed. Temperatures dropped into the upper 30s recently and I went out and started the Merc. Exactly the same hunting idle until the engine warmed up. I can eliminate the erratic running with the slightest pressure on the pedal--less than 100 rpm, but it returns once I release it, at least until the engine partially warms up. So it appears the linkage is not the issue, at least not in this case.

Dieselbenz1's suggestion of the water temperature sensor will be the next avenue of investigation, along with LiquiMoly's Diesel Purge.

Anyone done a purge on the 606 in the 95 E300D? Any suggestions or directions? I've watched a couple of run throughs on YouTube, but none for the 606. I've got a large canister for the two cans of purge and some hose. Is this done from below on the E300D? I'm assuming I'd be able to access the necessary connections by removing the windshield washer fluid tank.

My guess would be the idle computer. Cold weather can crack already brittle solder joints.

Wouldn't a cracked solder joint just render the idle computer totally ineffective, regardless of the outside temperature?

On my 1998 E300, I had lumpy idle, but only on a warmed up engine, and only with no load. Even just turning on the A/C compressor was enough load to make the engine instantly smooth. Turned out to be worn out IP. I replaced the IP and that solved it.

i suggest a compression test if you haven't tried that yet. Also can switch injectors around to see if there is a bad one or a dead cylinder.