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1986 MBZ 300SDL Fuel Injection Pump Problem

The Fuel Injection Pump had to be replaced in my 1986 Mercedes 300SDL. Since the pump replacement, the car shakes at idle, “quivers” during acceleration, and mileage is now 26mpg versus the prior 34mpg. Another strange thing, is that once the car is driven to a place that is 7000 ft or more above sea level, the engine continues to run, but there is absolutely no acceleration. I actually had to get out and push the car out of traffic while in Santa Fe, NM, and the car was still running. Santa Fe is over 7000ft in elevation. By the time I had it towed back to Albuquerque, it was back to accelerating, albeit shaking and quivering. I have been researching this issue for months, and I and 18 dealerships, and 4 independent mechanics have worked on my car for over a year to resolve the issues, but no one has been able to resolve the problems. Prior to instalation of the replacement Injection Pump, the car was smooth as silk, and would effortlessly accelerate to over 90mph getting 34mpg freeway, and at 90mph you could barely hear the engine. Now, the car shakes at idle, quivers throughout the acceleration process, and struggles to get to 90 (not that I drive that fast regularly… btw). The replacement pump has been repeately removed and analyzed, recalibrated, and bench tested, and it all works within specification. All injectors have been bench tested and were all operating at 100%. Many other engine sensor and vacuum tests have been conducted and are perfect. Is there anyone at NPR’s car talk who knows what is wrong with my car?

Just a guess - might a vacuum line be off/cracked/incorrectly connected?

It is worth a repeat analysis. The first three times vacuum lines were tested, no problems were found. The fourth mechanic that tested them found one line had been connected wrong, and the result was that the airconditioning began working perfectly for the first time in over five years. Maybe there are others still connected wrong … it is definitely worth a try. Thanks

You also might post this over at But having had 22+ mechanics look at it makes it quite a challenge.

Here’s an obvious one - fuel filter?

Thanks, I’ll try that website also.
All the fuel filters are clear.
One mechanic at Crown Mercedes in Saint Petersburg Florida, has the same model with exactly the same issues after replacing the injection pump … he has not found a solution either (he wants me to contact him when I finally solve this). Whispell’s Foreign Auto Service, Saint Petersburg, FL, contacted Mercedes Technical Support in Germany, and received no response from them. Orange County Benz, Costa Mesa, CA, was the mechanic that found one incorrect vacuum connection and now I have A/C again.

Yikes, desparation time! Has anyone swapped in a known good pump?

Technically, the answer to your question is no. I have not run into another '86 300SDL where I could ask for such an “organ Transplant” experiment. I also fear asking someone who’s car is running correctly to take a chance that removal and reinstallation could permanently ruin the way their car runs. My replacement pump has been taken off twice, recalibrated, bench tested, and verified as functioning correctly. The Crown Dealership mechanic has had the same problems, and his pump operates perfectly also. I suggested trying a brand new pump 6 months ago to the mechanic at Fletcher Jones in Vegas, but they all said this would not make a difference after reviewing the extensive history of tests and checks performed by other mechanics. A new pump is also $2,800, and my rebuilt was originally only $1,500. I am not a mechanic, but, because of the elevation issues, I can’t help wondering if it is some kind of a rarely-dealt-with sensor or adjustment issue. The car is very reliable as I have driven it across the US 2,800 miles each way twice in the last 9 months, it just runs badly at lower elevations and not at all in higher elevations. Another thing, it produced enormous clouds of black smoke at the elevation of Yellowstone Park (so much smoke that the rangers politely asked me to leave and come back after my car got repaired), but ran fine going past the Tetons on the way to Pocatello, Idaho. The dealership in Pocatello, could not find anything wrong. On the way back, Sioux Falls, SD, Mercedes could not find anything wrong either.

Keeping in mind that I’m not a Benz expert but having experience with VW diesels, is it possible the pump timing is incorrect?

This can cause sluggish performance, rough idle, poor fuel economy, etc. and the slightest tweak in the pump can throw the pump timing off by a mile.
In the case of VWs, one cannot install an injection pump and call it good. There are special tools and procedures for setting the timing after the pump is in place.

And naturally being curious, what was the reason for replacing the pump on a good performing, running smooth as silk engine in the first place?

Thank you for taking time to suggest a fix, I appreciate any suggestions. Whispells Foreign Autos of Saint Petersburg, FL; Fletcher Jones of Las Vegas, and Orange County Benz have all worked on “perfecting the timing” and claim this could not be the problem. They all do great work, so, I am assuming the timing is not an issue. Whispell’s Foreign Auto did procure one tool they did not already have, but they are a two generation shop that has an outstanding reputation (Whispells has actually worked on my family’s cars since the 1960’s). When I asked Crown Mercedes of Saint Petersburg to just check out my car, they told me they could not best any work done by Whispells.

The reason the pump had to be replaced is that 500 miles east of Mobile, Alabama, the original injection pump literally exploded. There are lots of little bearings inside the pump and they flew apart so hard that they ended up shooting through the casing of the original pump. The car was then towed to Mercedes of Mobile, where they claimed the engine block had exploded. They then offered to buy my car from me. So, I got on the phone to Whispells in Saint Petersburg Florida, and explained what I was seeing in the engine compartment to Seldon Whispell. I put Seldon on speaker phone, and the mechanics gathered around at Mercedes of Mobile while Seldon explained to the mechanics what was actually wrong. I then had the car shipped back to Saint Petersburg Florida to Whispells, where a rebuilt, re calibrated, and tested injection pump was installed. Seldon then worked on perfecting the timing. Once I took delivery, the car has never run right again. Over the last year, I have driven the car alot, and it is totally reliable in lower elevations, but it runs terrible in comparison to the way it used to run. Seldon Whispell has spent hours and hours taking a closer look at the engine to make sure no bearings could have penetrated any other part of the engine, and confirmed that nothing else is wrong. The most interesting part of all of this, is that a mechanic at Crown Mercedes of Saint Petersburg, Florida, has the same make model and year. He also had to replace his fuel injection pump, and his car now has the exact same performance problems that mine has. This mechanic said he did not know how to correct it, and we are staying in touch incase I find out how to fix it and can tell him how to fix his also. I followed “TEXASES” recommendation and posted my question on
Another stange thing is the car’s performance at higher elevations as noted in the above comments.
I originally learned to drive on a stick VW diesel Golf in the Netherlands… what a great car!

Before adding any more comments let me ask about the exploded engine block claim. Was it blown up or not? If so, does the car have a different engine?
A blown engine block is not a guessing type of diagnosis and maybe someone there was mistaken or lying in an attempt to get you to unload the car on the cheap.

Other than that, and if the pump timing is known to be correct without a doubt, then the things I would look at would be:
Engine compression.
Dragging turbocharger or intake leak.
Injector fault due to debris.

My personal opinion is that when it comes to diesels (and gas engines too) compression should always be verified whenever an engine performance problem exists. One should always weed out the possibility of a mechanical fault from the get-go rather than risk spending a lot of time and money throwing parts at something that will never be cured by that practice.

As I said, I’m not a Benz expert but they use Bosch diesel parts just like the VWs use and the principles are the same.

Some more guesses, after you follow OK’s good advice: have the valves been adjusted? Is there any possibility of a bad cam chain or chain tensioner letting the cam get out of time? Any way to have a fuel pressure gauge plumbed in to watch fuel pressure while on the road?

The engine block was fine, so the engine is still original. The mechanics at Mercedes of Mobile were young and had not worked on my model car before, so Seldon Whispell had a great time teaching them on speaker phone about the 1986 6 cylinder mercedes diesel.

Engine compression was tested and is fully within standard specifications. The bottom half of the engine was rebuilt by Downtown LA Motors Mercedes in 1995, and Whispells retrofitted a new genuine Mercedes head to the engine in 2007 before it became a problem (the originals were prone to cracking because they were machined out too thin for long term endurance - a rare mercedes engineering design error)

I will have to ask about dragging turbo charger and intake leak, although the turbo was tested just after pump replacement a year ago.

Injector fault due to debris - all injectors were bench tested on two separate occasions since pump replacement and even the slightest suspicion got my approval for replacement with new.

Car is being shipped to CA from FL on July 6 - Next stop, Orange County Benz in Costa Mesa, CA. I will be passing along all these suggestions to Ron (the diesel mechanic) at OC Benz.

I asked Seldon Whispell about valve adjustments and he said the 6 cyclinder diesel valves are self adjusting versus the prior generation 5 cylinder that needed adjustment.

I am shipping my car to OC Benz in CA, and will follow up with the diesel mechanic there, Ron, about CAM chains and tensioners. Puel pressure guage is also an interesting diagnostic process I will mention.

Went to Netherlands on business recently, and talked with a few taxi drivers who were driving my make model and year of car. Their cars had much more mileage (kilometerage) than mine, and ran as new. I asked them questions since many worked on their own cars, but they all said they would be happy to take a look at my car for me … (harumpf). Does anyone have access to the European information networks on diesels?

Any chance the compression test readings were jotted down for you (they should have been).
Just wondering what they are.

The reason I ask is because people post on this forum all of the time with car problems and refer to (in a nutshell) the “compression being good”. A followup usually reveals figures that are horribly low. What’s even more appalling is the number of shops who dish out lousy readings and consider them good.

The only way I can figure this logic is due to the proliferation of badly incorrect info that is printed in service manuals, including factory manuals. This is also not limited to compression readings; it also applies to oil pressure, ignition timing where applicable, and any one of dozens of other technical areas.

Go to a parts house or a public library and thumb through manuals like Chiltons or Haynes for compression readings. The info they give is so bad as to be downright laughable.
I’ve even mailed publishers like this in the past and asked who is in charge of verifying the info they provide. So far, they ain’t talking.

As to your diesel my guess is the compression readings should be in the 550ish PSI range. Based on the VW diesels once they get in the 400ish range they’re on the downhill slide.

Here is an important number you have left out: How many miles on this car?? How much money have you spent attempting to repair it??

The car currently has 212,256 miles. $4,437 so far excluding shipping/towing costs. To give you an impression of the care it has received, it still has the original flawless interior, and original shiny paint job.

The car is being shipped to Irvine, CA, on July 6th and I will have OC Benz confirm the compression. I will be surprised if this is the issue since Fletcher Jones of Vegas said the compression was “as new” September 2009 @202,000 miles … but, they also could not figure out why the A/C would not work … so it is worth rechecking.

It just seems strange to me that another 300SDL owner (the mechanic at Crown Mercedes in Saint Petersburg, FL) had to replace his injection pump, told me his compression was fine along with many other things we discussed, and yet his car has the same problems as my car.

Why did you exclude shipping and towing? Looks like 5 large and no end in sight…Never fall in love with something that can’t love you back no matter how shiny the paint is…You should have kissed it goodbye when that first pump scattered…

including all the shipping and towing to date it is over $6,600!

…it is said that marriage is grand, but divorce is a hundred grand.
(obvioously this old saying is not adjusted for inflation and housing bubbles)

Hind sight is always 20/20. I doubt any rational person with an IQ above 60 could have imagined that a standard 6 cylinder diesel engine that has powered thousands of vehicles over millions of miles for over 25 years would exceed current technical know-how. Unless OC Benz can fix my car, I plan to let everyone in the Mercedes Club of America know that when a diesel injection pump goes, crush the car.

Of course there is always the somewhat curious notion that Mercedes wants all these older Mercedes off the road, because as long as they are running, the owners won’t buy a new one … and all of the new cars I have had during the many years I have owned my old 300SDL, have long since succumbed. Curious … aint it!

Today, the people in the service shops are not even on the same planet as the ivory-tower engineers who design the cars and their powertrains. Serviceability has NEVER been one of Benz’s trademarks…Good luck and please let us know how this saga ends and what the final tally is…

I submit my comments based on complete and total ignorance of a Benz turbodiesel. I own an older VW diesel and sometimes follow a few VW diesel web sites and have not heard of a problem such as yours. I wonder if the problem is traceable to the turbo or its controls. A non-turbo, completely mechanical diesel is a fairly simple thing to keep running well. A few problem areas regarding starting and running a non turbo mechanical diesel are glow plugs, compression pressure, pump timing, cam timing, fuel line air leaks or rarely, fuel quality. Who can know what problems the complication of a turbo can make?

Does your engine have some electronic control that could be partially failed?