Hello Forum community. Hope someone can point me in the right direction.

I have a 1985 300sd.

The car runs Ok until it is hot.
When it gets hot then it idles very high.
If I push the throttle then it revs even higher and is very slow to rev down.
It even goes so far as to keep the throttle high when driving. Sorta like cruise control. But, the main characteristic is that it revs high when hot and it takes a long time to rev down.

It is a hard start when hot also.
When I first start the car in the morning it does NOT do this.
Is this from leaky injectors (at nozzle end), rack damper, blowby (piston rings worn).
Help me!!!!!

Thanks goldveg
.

The spring that returns the throttle linkage may have come off or broken. Locate it close to the fire wall just below the shaft from the throttle pedal.
Clean all the linkage points and re-grease.
Try it without the cruise control hooked up.

__________________
1984 300SD turbo 126
"My true love"

God made me an atheist and who am I to question His wisdom

Have the exact same issue. It does that after I drive it hard and put it in park goes from the normal 750rpm to 1000rpm. It has nothing to do with the coolant temp in my case but more outside temp? Maybe the hot oil or fuel is doing something inside the pump? I'd love to figure this out too.

EDIT: it also seems to do this frequently in the summer. again, nothing to do with the actual engine coolant temp. i got off the tollway today and it was idling at 1000rpm with the a/c compressor on. engine at 80c. idle went back to normal after driving on some neighborhood roads...

__________________
83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

The rack damper bolt doesn't have anything directly to do with idle speed.

As suggested by The Gears this behaviour can simply happen due to poorly lubricated throttle linkages.

Idle speed is controlled by the governor inside the injection pump. Wear inside the governor can cause idle speed to remain high a bit longer than it should, when the throttle is released. The governor is not adjustable.

Also, it is not particularly unusual for the idle to remain a bit high immediately after you exit the highway. Also some cars idle roughly for a minute or so, after extended highway use.

Goldveg, from your user handle I assume you are running vegoil. It would be helpful to know if you are starting and shutting down on diesel, then switching to veg, or....? Alternative fuels complicate some issues.

__________________


Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Both posters should make sure the return line from their injection pumps are flowing fuel when the engine is running. It aids in keeping the internals of the injection pump cooler. If fuel supply pressure is low the relief valve will stay closed.

That may not be the problem both of you are experiencing is pretty certain. At the same time it should be returning fuel to the tank. Easy to check and rectify if not doing it. In both your cases I would want to know it is working.


Another function of the relief valve is to maintain a constant and adequate fuel feed pressure in the base of the injection pump.

I am not personally aware of any previous variable idle speed complaints from a return deficiency of any sort. The return system was designed to have function at the same time so it should be working. Many are not working properly. Either the relief valve is tired or blocked or the supply pressure is too low. Also have a look at your primary filter for air bubbles being in transit through it. Both when starting with a cold engine and after the engine is warmed up. There should be none present other than the normal stationary one.

If on alternate fuels the feed temperature of them can be a factor I suspect.

Most of the Fuel Injection Pump is lubricated by the Carnkcase Oil From the Engine. Only the Fuel Injection Pump Elements get any lubrication from the Fuel.

You did not comment on this but I will include it here. The is backed off/out and then the idle speed is adjusted. After that the Rack Dampner/Governor Idle Pin.

The Rack Dampner/Governor Idle Pin is not supposed to effect the idle speed much. Meaning that you idle adjustment should not be hanigng on the Rack Dampner/Governor Idle Pin.

Besides the sticking Linkage Balls, Accelerator Pedal Linkages, The Bell Crank on the Fire Wall there that Plastic Cam on top of the Valve covor.

At the Bottom Rear of the Fuel Injection pump is another Springt that goes to the Throttle Arm and a Bracket that Bolts onto the Block.

__________________
84 300D, 82 Volvo 244Gl Diesel

Not exactly true. On the MW Fuel Injection Pumps there is an adjustment inside of the Governor for the Idle Speed. However, it requires that you remove the back cover of the Governor.

Over on Superturbodisel ForcedInductions had some pics showing the adjustments.
However, fooling with those adjusments is a least resort and done at your own risk.

__________________
84 300D, 82 Volvo 244Gl Diesel

I've checked my linkage and everything is good. I'm swapping my pump sometime soon, so this should go away. It always returns to idle quickly when it isn't doing it's high idle thing. I will check my fuel flow of the lift pump at the return line. I am almost positive the tank strainer is very nasty. But I have no lack of power issues and plenty of black smoke if I floor it and the turbo hasn't spooled up

__________________
83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

There are two return springs.

One on the bulkhead attached to the pedal rod.

One at the rear of the injection pump, attached to the engine.

All of the throttle link rod sockets/pivots need regular lubrication, or they will bind and drag.



.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

Normally one would expect the linkage not to show much difference with the engine compartment temperature changes. Actually tending to be more sticky when cold would be my choice . Although there could be an exception.

This is what made me think although it is so common that it may not be the issue with this one. That said it would still be nice if it were.

With no fuel return the internal core area of the pump will rise in temperature more than with the fuel return carrying away heat. If enough in some kind of marginal situation to tip the balance has not been mentioned on site.

If the car has had a diet of wvo I would do an internal soak and flush of the pump with laquer thinner before replacing it as it is so cheap and easy to do. Again I would not expect it to solve the problem. Or the odds of it doing so are very poor. Still I would. Perhaps other than this issue any car burning wvo and having no rubber components in the injection pump should probably be flushed as a maintenance item. I am not saying this car has or is on wvo. In the past it has been mentioned on many threads very late by the poster and was the cause of the given issue.

That makes total sense to me. I'll check the flow of fuel next. I don't see much fuel flowing through the pre filter when it's running so hopefully that's it

__________________
83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

well, i ran both lines into a temporary bottle with diesel under the hood and it made no difference. how much fuel shod be coming out of the return line at idle and and at about 2500 rpm? also how quickly should it use a water bottle of diesel at idle?

__________________
83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

On a healthy fuel supply system there should always be some. When you say it made no difference was there fuel coming out the return line in your test?

I suspect you mean the engine acted the same as when the lines where connected though.

The reason for the return flows presence . . The average old relief valve if still regulating opens at around 10-12 pounds fuel pressure. The lift pump in good shape on your model with fairly decent fuel filters provides at least twenty pounds and usually more fuel pressure.

The volume of fuel at the elevated pressure beyond the 10-12 pounds is returned to the tank. Some relief valve springs have aged to death and are opening at far too low a pressure.

The manufactures reason for this setup is to make sure there is always a stable fuel supply at a desired pressure available in the injection pump. Plus incidentally or intentionally core heat generated in the injection pump is reduced by the constant fuel circulation.

There was a pulsing flow of fuel coming out of the return line on both the test bottle and when connected to the fuel tank. I did stretch the spring on the outlet of the IP a few months ago. I forgot the length I stretched it to but it was a safe length a few people recommend on this forum. Is there any benefit to having more lift pump pressure? Let's say 15 or 16 psi? Will it blow seals in the IP or damage it?

__________________
83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

The Overflow/Pressure Relief Valve is what controls the pressure Fuel Pressure that is inside of the Fuel injection Pump Housing assuming that the Spring inside of the Fuel Supply/Lift Pump is not broken.

The Max Fuel Pressure on My Year and Model is I belive 18 psi.

I can't remember if it was funola or someone else but one of our Members made and Adjustable Over Flow/Fuel Pressure Relief Valve and offered them for Sale about 2 years ago.

__________________
84 300D, 82 Volvo 244Gl Diesel