M103 stumbling intake backfire - PeachParts Mercedes-Benz Forum

DrewZ926 · #1 02-14-2023, 07:34 PM

Hey guys i've got an 89 190e 2.6 5spd that i've had for a few years now. I bought it not running (rotor hat was busted). I replaced the cap and rotor, new plugs (non resistor) and wires. Ive always had some low rpm stumbling. Not all the time but just at random moments. Sometimes it'll stumble and lately ive started to hear a backfire from the ignition side. When i first start it it'll fire up no problem at all but it's pretty rough for a moment til it smooths out. I've replaced many items and it has honestly ran pretty well otherwise. Seems to have decent power for what it is. Since the other day though i have no power once i go further than half throttle. It just kinda bogs down it seems. Very slow acceleration and it's not really driveable at this point. I'm hoping somebody can point me in the right direction?

parts i've replaced since i put it back on the road:

-rotor and cap
-wires
-plugs
-o2 sensor
-ovp relay
-iac valve
-coolant temp sensor
-fuel pumps and filter
-vacuum lines and intake hoses

A lot of this was kind of guessing and throwing parts at it but i also figure it would be good to go ahead and do basic maintenance for the future since a lot of it seemed original.

Jorn · #2 02-14-2023, 08:58 PM

My 300SE has low rpm stumbling when I use 87 instead of 93 fuel.

Sugar Bear · #3 02-14-2023, 09:16 PM

Check for air leaks in hoses and the boot under the fuel distributor. Stumbling and backfiring are caused a lean mixture which can be caused by a vacuum/air leak. A smoke test is a good way to find vacuum leaks.

lsmalley · #4 02-15-2023, 09:36 PM

I would replace the fuel injectors as well. I had an issue similar a year ago and honestly what've now decided to do is, not necessarily throw parts at it, but if I find that there is an issue with say the fuel pump, then if the whole system is like 30+ years old then it all gets replaced. This isn't the same as throwing parts at it as you would already know what the issue is, but instead of just say getting new plugs, replace the entire ignition system like rotor, cap, and wires as well. On a side note, you might want to try tapping the airflow potentiometer on the side of the throttle body with a long socket. sometimes if that gets stuck or is off it will give the same symptoms that you are describing.

Sugar Bear · #5 02-16-2023, 09:21 AM

lsmalley, Your method can also be viewed as high level maintenance/preservation since some of the parts you replace now may not be available in a couple of years when they fail. My method used to be on high mileage cars was if a part was off for any reason and was a wearable item e.g., a belt, hose or bulb it got renewed. I think this upped reliability and in the long run saved time...my $0.02.

lsmalley · #6 02-16-2023, 12:40 PM

Quote:

Originally Posted by Sugar Bear

lsmalley, Your method can also be viewed as high level maintenance/preservation since some of the parts you replace now may not be available in a couple of years when they fail. My method used to be on high mileage cars was if a part was off for any reason and was a wearable item e.g., a belt, hose or bulb it got renewed. I think this upped reliability and in the long run saved time...my $0.02.

Yes. But also, I wouldn't toss out the non-failed part, I would keep it on standby for either future testing against any issue that arises in the future that may potentially involve said part. It was after I had the issue that ended up costing me to miss 2 weeks of work is when I decided on using my method

. Definitely a lot cheaper, for me at least, to do it that way.

Sugar Bear · #7 02-16-2023, 05:32 PM

Yup, good used stuff gets saved.

Dave Kost · #8 02-18-2023, 11:05 PM

Drew,

Check your ignition coil, or replace
Check for an exhaust blockage.
Check the idle switch

Check the vacuum line going to the EZL. The EZL advances the ignition timing as RPM's increase. If the EZL isn't working or isn't getting the proper vacuum signal the engine will have a hard time advancing rpms under load.

If you haven't tested your fuel pressure, I would do it now. Additionally, I would keep the fuel gauges hooked up, and while the engine is idling rev the engine. The system pressure should recover to spec immediately with a minimum drop in pressure. If it doesn't the fuel pressure regulator needs replaced.

CIS injection needs constant fuel pressures to work properly.

Keep us posted. (It could be a combination of the above items)

DrewZ926 · #9 02-19-2023, 07:58 PM

Thanks for the responses guys. I've visually inspected everything and everything seems alright. I ordered a smoke leak tester and i'll be doing that in the next few days. I drove the car today and had no issues at all though. Since the issue can get worse and really affect drivability i've lost some trust in the vehicle so hopefully i can figure this one out.

Duke2.6 · #10 02-22-2023, 11:53 AM

Quote:

Originally Posted by Dave Kost

Drew,

If the EZL isn't working or isn't getting the proper vacuum signal the engine will have a hard time advancing rpms under load.

A couple of points - if the EZL "isn't working" there will likely be no spark. Vacuum advance is only applied at light to medium load and has no effect at high load. No vacuum advance will increase fuel consumption, but decrease emissions.

The rate of advance with revs is controlled by the R16/1 resistor. OE is 750 ohms on US models. Removing the resistor will increase the rate of advance with RPM and improve low end torque.

Shorting the R16/1 resistor connector will yield the slowest rate a above advance, which will further reduce emissions, but the car can barely get out of its own way below 2000 revs. This may not be that noticeable with automatics, but it definitely is with a manual transmission.

If you are having problems passing emission tests with a M103 block the vacuum advance signal line and short the R16/1 resistor plug

Duke

DrewZ926 · #11 02-22-2023, 08:15 PM

Well i did some smoke leak testing and didn't find anything leaking. I'm kind of suprised but the only place that was leaking was the vacuum line going to the fuel pump regulator but that doesn't mean anything.

nulu · #12 02-22-2023, 10:54 PM

check and replace EHA, it gives acceleration enrichment

Dave Kost · #13 02-23-2023, 10:08 AM

Do this.......

If you haven't tested your fuel pressure, I would do it now. Additionally, I would keep the fuel gauges hooked up, and while the engine is idling rev the engine. The system pressure should recover to spec immediately with a minimum drop in pressure. If it doesn't the fuel pressure regulator needs replaced.

Duke2.6 · #14 02-26-2023, 01:46 PM

I didn't see that you tested the fuel accumulator. It's an often overlooked part of the KE system. Mine failed and there's and easy on-the-car test to determine if it has an internal leak. Suggest you read the following thread. The test is in post #11.

M103 starting problem

The following is the post mortem failure analysis report for the FA.

KE fuel accumulator destructive failure analysis

Duke

3DawgDoug · #15 03-08-2023, 11:19 PM

I don't know where you stay but check your Cold Start Injector. Mine was plugged and would not deliver fuel. The engine would catch and run for 10-15 seconds and die. You pull the fuel line loose and the two socket head screws that hold the injector in place. Pull the injector, loosen the FD end of the injector pipeline, and pull the line up so it can be connected to the injector. Jump out your fuel pump. Have someone turn the ignition on and see if the injector delivers fuel.

I'm in North Central Florida and yes, the cold start injector we needed to get a reliable start, even at 75 deg. F.