Need M104 diagnostic help - E320 cabriolet

[AMG5PT4]

My brother has a 95 E320 A124 Cab. A few months ago, the car began to show zero throttle response generally when the car is below full operating temp and stop and go. I visited over Christmas and drove the car and experienced the issue first hand. You could push the go pedal to the floor with absolutely zero response from the engine other than a slight bogging down (I assume because of the throttle opening). The issue is intermittent - sometimes it happens, sometimes not.

Over the past couple of months, here is what has been replaced:

• Fuel pump
• Fuel filter
• Rubber fuel lines at the fuel pump
• O2 sensor
• Bosch MAF tube, NOS
• Fuel injectors, rebuilt
• Spark plugs
• Coil packs
• ECU was swapped with one from Jono, no change

The ETA (Don Roden) was replace about 4 years ago
Main harness was replaced previously
Lower harness (rebuilt) was replaced 4 years ago

The car has no CELs and when hot does not display this tendency. Really unsafe coming off the line with zero throttle response and moving forward only from idle engine speed.

Any thoughts appreciated. Jono wants to replace the rubber fuel lines into/out of the IM as he says these lines corrode from the inside. We have not attached any fuel flow meters or any other diagnostics. Essentially, we have thrown parts at the car. My brother had most of the work done by a local Tampa, FL MB shop and they are scratching their heads.

Car has 75k miles on it.

 

[Alphasud40]

Does this A124 cabriolet have ASR?

 

[AMG5PT4]

No

 

[Alphasud40]

You need to find a shop that isn’t afraid of good old fashioned diagnosis. I would do a search and see if you can find another shop. A shop that has a chassis dynamometer so they can easily replicate the symptoms. If it was in my bay I would first start with the basics like monitoring airflow and fuel pressure, looking at O2 sensor voltage, monitor TPS voltage, look at the NSS input that kind of stuff. You can still do the same diagnostics without a scan tool it just takes longer.

 

[gerryvz]

So my first question would be: did this situation start immediately after any previous work was done, or did it just appear out of the blue? That should provide a valuable clue if the answer is “Yes”.

Is it idling OK when started up cold? Fires up right away when cold? Fires up right away when warm?

What is the build date on the main (upper) harness? I know you said it’s been replaced, but you should check the date it was built, on the white label near the battery.

The very very first thing I would check on this motor is the fuel pressure regulator, located on the fuel rail. There’s a HOW-TO here specifically for this engine (same part/procedure for the M119 too).

There’s another fuel pressure check valve at the fuel pump (or pumps, depending on the car’s VIN — early ones had two Bosch pumps, later ones had a single Pierburg fuel pump). That check valve is a cheap part and should be checked/diagnosed — a new one may have been included with the pump(s).

Also I would monitor fuel pressure at startup and while running to see if there are any deviations from norm. Easy enough to do per the fuel rail.

Also check that fuel pressure leak down after the engine is shut off, meets factory specs in the FSM.

Has the throttle linkage and cable been physically inspected for smooth operation all the way from the gas pedal to the ETA?

Secondly, I would check blink codes to start off with. Then clear, drive 50 or 100 miles, and re-check. Has the shop bothered to do this?

Thirdly, I’d check plug wires both for resistance and for physical connection at both the plug and coil pack ends. These wires can and do go out.


Other considerations

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Fuel injectors even rebuilt can still be bad. I had a set professionally rebuilt for a 104 and they were unsat. Not likely, but a possibility. I maintain a couple of spare sets of used 104 FIs to swap in if ever needed for diagnosis. Not likely an issue, though. But your brother could swap the old injectors back in to see if the problem goes away (if other diagnoses don’t pan out). It’s one variable to eliminate.

Sometimes, a bad coil pack will show up in blink codes, other times not. But it is a common failure. Generally if you have a bad coil pack you will smell unburnt fuel in the exhaust. It’s worth a sniff, though I know that exhaust is rich at startup until the cat heats up. So, I’d sniff when the engine is warm.. I’ve had coil packs go out on every 104 I’ve owned. You can swap them around to isolate a bad one if you suspect one is not working.

I’m also wondering if the crankshaft and camshaft position sensors have been tested to ensure they are good. Coolant temp sensor too (the one that provides input for the HFM system).

Air filter been replaced?

I agree with Alphasud. Sounds to me like the shop is not doing some basic diagnosis and isolation work. I mean, it has to either be fuel or spark.

 

[AMG5PT4]

Thanks for the replies. Some answers:

The issue started out the blue and did not commence after any service.

Cold and hot start-up and idle has no issues

Blink codes - none

The main harness has a 2006 date

My brother wants to bring the car to Blue Ridge for further work and diagnostics. Just such an odd issue, but dangerous! The FPR on the rail has not been replaced.

Will advise.

 

[TerryA]

My wife had a similar experience with our 84 500SL and the same happened to our 86 300E. No throttle response. If I remember correctly in both cases it was a crumpled plastic ball in the throttle linkage. For sure on the 500SL. In its case she phoned me that she was getting off the freeway and thought that the transmission failed. She said it was running but wouldn’t move in any gear.

After it was towed home I discovered the $2 or $3 part deteriorated. I’m pretty sure the 300E had the same problem.

BTW, your E320 may have a similar linkage system.

 

[gsxr]

Since the issue is temperature based, I'd start by viewing live data from the HFM system (via SDS HHT-Win) and verify the reported coolant temperatures are correct. I'm assuming these will be ok, but it's worth verifying.

 

[R129 UK]

My wife had a similar experience with our 84 500SL and the same happened to our 86 300E. No throttle response. If I remember correctly in both cases it was a crumpled plastic ball in the throttle linkage. For sure on the 500SL. In its case she phoned me that she was getting off the freeway and thought that the transmission failed. She said it was running but wouldn’t move in any gear.

After it was towed home I discovered the $2 or $3 part deteriorated. I’m pretty sure the 300E had the same problem.

BTW, your E320 may have a similar linkage system.

My immediate first thought too Terry, @AMG5PT4 when this issue occurs can the engine be revved "by hand" from under the bonnet?

Also. does it misbehave until either the engine gets to operating temperature or it is shutoff and restarted etc.?

Do you have a decent window of time to diagnose if it starts playing up?

 

[R129 UK]

Since the issue is temperature based, I'd start by viewing live data from the HFM system (via SDS HHT-Win) and verify the reported coolant temperatures are correct. I'm assuming these will be ok, but it's worth verifying.

OP states it is intermittent but does occur before fully warming up - the worst of all worlds to diagnose.

 

[AMG5PT4]

No, have not looked at the throttle linkage for mechanical issues. Not sure if an intermittent problem would show as a mechanical failure in the linkage.

I need to get the car to Jono.

 

[gerryvz]

Fuel pressure diagnosis while starting, running, and stopped would be one thing.

Checking the plug wires would be another.

And as mentioned a quick hand check both underhood and observation with a helper pressing the pedal, of the throttle linkage would be all good ideas.

 

[R129 UK]

No, have not looked at the throttle linkage for mechanical issues. Not sure if an intermittent problem would show as a mechanical failure in the linkage.

I need to get the car to Jono.

Any update on this issue?