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  #1  
Old 02-14-2021, 01:19 PM
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W124 300TE warm running weirdness

Hello all!! Newly acquired 1988 300TE is causing me a little bit of a fuss. Some background of the car, was sitting since 2004 inside a greenhouse of all places. Relatively rust free, with 158k miles. I rebuild the fuel distributor with new seals, new cap/rotor/plugs, new fuel filter, new rad, and a whole bunch of replaced fuses (corrosion from sitting in a greenhouse). The car doesn't have a cat anymore, the exhaust was chopped up and in the trunk when I got it, so I made a new exhaust minus a cat (but still has an O2 sensor).

Was on my shakedown run and made it a couple laps near my house, about 11 miles. Decided I felt good enough to venture to the gas station and put some non ethanol 91 good stuff in it. Fired right up and made it just about home then it decided to tell me "not so fast buddy". Anyways, the symptoms its having now is it will run very smooth up until just below the 80C temp point. It seems to have a little idle "surging" but I use that word with reservations, because it just sounds similar to when an AC would kick on or if you were to hook up jumpers to start another car and the alternator recovers. It has a hard time starting up when warm, which I would attribute to the injectors possibly, but normally it'll start up in about 5-8 seconds when warm. If I let it sit overnight and start it the next day, it fires off on half a rev. Once it does start when warm, it will sit and idle fine, then if I give it full throttle, a blip, it'll just pretend that I didn't even do that. It'll rev a little then sputter, then return to idle (with my foot still pressed down). I checked Idle control valve, works as it should. Injectors were cleaned in an ultrasonic cleaner. I can get it to rev up when warm if i do some form of tap dancing on the pedal, like full throttle, let off, full throttle, let off, ect. I took a video to convey that as its sort of hard to explain: https://youtu.be/O4b-X8bmy7g yes i know the exhaust sounds horrible at 3k rpms, theres a hole somewhere I need to fill.

What seems odd is when I was driving it on my laps, it was running flawless, and had tons of power (keep in mind I'm used to w123 300TD power level) and didn't have any weird throttle input things going on.

Thoughts appreciated on if I should look at the fuel distributor again(I've rebuilt a few with success so not super worried here, but never know), or check out a coolant sensor that could be making it do weird things when warm. No check engine light present. I've seen some posts about an ignition coil being bad when it warms up, but I am not one for just throwing new parts without reason. Thanks for taking the time to read

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Old 02-14-2021, 03:30 PM
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In addition to the coolant temp sensor, the O2 sensor and the air-flow potentiometer are components involved in acceleration enrichment, not to mention the EHA.
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Old 02-14-2021, 05:54 PM
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Good call. I did have to re wire the O2 in when I recreated the exhaust. Iíll take a look to see if anything came undone. Then once confirmed check if behavior changes once warm and I unplug the O2 plugs by the passenger seat. Will report back
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Old 02-14-2021, 09:43 PM
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Originally Posted by Frank Reiner View Post
In addition to the coolant temp sensor, the O2 sensor and the air-flow potentiometer are components involved in acceleration enrichment, not to mention the EHA.

Okay O2 sensor was hooked up still, I'll have to check its functionality via diagnostic port or bench testing I've seen is an option too. Not sure exactly how, but I can use the search feature for that. While i was out testing, I unplugged the EHA and the car's behavior didn't change at all when warm which I presume is to be expected? Unplugged both EHA and O2 sensor with no change in behavior.
Is there a way to take the air flow potentiometer off and clean the contacts off with a pencil eraser? I did that trick on my SL's TPS and it seemed to help (electronic djet). I have messed with adjusting the EHA before on my 126 but that was because it ran horrible when I got my hands on it. My 124 runs well in comparison to what that was like, so I am a little hesitant to adjust the EHA and cause adverse effects.
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Old 02-14-2021, 10:18 PM
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Originally Posted by ajschank View Post
Okay O2 sensor was hooked up still, I'll have to check its functionality via diagnostic port or bench testing I've seen is an option too. Not sure exactly how, but I can use the search feature for that. While i was out testing, I unplugged the EHA and the car's behavior didn't change at all when warm which I presume is to be expected? Unplugged both EHA and O2 sensor with no change in behavior.
Is there a way to take the air flow potentiometer off and clean the contacts off with a pencil eraser? I did that trick on my SL's TPS and it seemed to help (electronic djet). I have messed with adjusting the EHA before on my 126 but that was because it ran horrible when I got my hands on it. My 124 runs well in comparison to what that was like, so I am a little hesitant to adjust the EHA and cause adverse effects.
If you unplug the EHA while running, with no change, it is a pretty good indication that the EHA is not receiving a signal (mA current) from the ECU. Check for a duty cycle at the diagnostic port.
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Old 02-15-2021, 02:50 PM
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Quote:
Originally Posted by Frank Reiner View Post
In addition to the coolant temp sensor, the O2 sensor and the air-flow potentiometer are components involved in acceleration enrichment, not to mention the EHA.
Quote:
Originally Posted by Frank Reiner View Post
If you unplug the EHA while running, with no change, it is a pretty good indication that the EHA is not receiving a signal (mA current) from the ECU. Check for a duty cycle at the diagnostic port.
My meter only does Vac, but i did manage to measure across ports 2 and 3 on the diagnostic port and these are my findings: Key on, cold 7.5Vac. It's 10 degrees today so getting into closed loop is proving challenging without driving. I did notice with the engine cold and I unplugged the EHA it would stumble a bit and idle down (today). Then upon plugging back in, the idle would go up to about 1200, then would normalize, and finally really low idle <500rpms sounding like it wanted to die. If I gave it throttle input it seemed responsive and fine but would return to that low rubble idle. I'm sort of stuck on the idea of how it was driving perfectly then just decided to do anymore. Thanks for your thoughts so far Frank! I can try to wrangle up a meter that will show me duty cycle if that is needed. I've got a spare EHA from my 126 that seemed to work fine, can swap that over to test.
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Old 03-21-2021, 09:52 PM
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Just as an update, still tinkering around, but possibly have an idea what might be up. It seems to run perfect for about 10 seconds when you first turn the car on, then it'll stumble and try to die, and if you give it throttle it backfires in the intake. After a bit, I noticed my tach stopped working which made me think, well maybe my rear CPS sensor is going bad on me. Have that on order, will report back my results! I did unplug the sensor that lives under the fuel distributor intake boot, and it caused my idle to surge to 1000 and back down, so that leads me to believe that it's working fine. So next might be my potentiometer, really hope its not that!

Updates soon!
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Old 03-26-2021, 11:01 AM
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Crank sensor replaced. Car fires up, runs perfectly smooth for about 2 minutes, drove around the yard and felt okay. Then it starts to break up/cough with any throttle input after a small amount of time. Grasping for ideas, I popped the distributor cap off to see if anything looked weird in there. To my surprise there was some corrosion or sign of arching on the contact areas. This is a brand new distributor cap and rotor, so not sure as to why this would arc/corrode so fast. Could the spark plug wires be the cause of this? I haven't replaced that those yet since they seemed to be okay. Could the coil also cause this type of issue? The plug wires could be original as they still have the plug numbering labels on them. What weird when it starts to die/cough/whatever its doing, if i turn it off and back on it'll run perfect again for a brief amount of time, then revert to same behavior as above. Any thoughts would be appreciated.
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Old 03-26-2021, 01:31 PM
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If the plug wires have high resistance they will burn the cap and rotor.
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Old 04-02-2021, 11:00 AM
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Swapped over my plug wires since I figured it was worth a go, and since they looked original might as well. No noticeable improvement. Got a mass air flow sensor from a previously running parts car and swapped that over, no improvement there either. It seems that when it’s cold and running it’ll run and idle fine for a bit until it gets a little temp in it. Today it was about 35 and it ran fine for about 5 minutes then started it’s normal deal. What seems to happen is it idles along happily, then I can hear the air flow really start to increase(or at least create what sounds like a sucking noise) near the intake, then it starts to idle down and spit and sputter on a couple cylinder then it will finally shut off. I’ve replaced all the hoses off the idle air control valve, and also have cleaned that valve. I’ve inspected the boot on the intake and that is supple and crack free.

So all I’ve got left to really try is the jetronic controller, which seems unlikely, and the sensor that sits on the throttle butterfly below the fuel distributor and all that. Any ideas appreciated as always!
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Old 04-02-2021, 12:43 PM
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Quote:
Originally Posted by ajschank View Post
Crank sensor replaced. Car fires up, runs perfectly smooth for about 2 minutes, drove around the yard and felt okay. Then it starts to break up/cough with any throttle input after a small amount of time. Grasping for ideas, I popped the distributor cap off to see if anything looked weird in there. To my surprise there was some corrosion or sign of arching on the contact areas. This is a brand new distributor cap and rotor, so not sure as to why this would arc/corrode so fast. Could the spark plug wires be the cause of this? I haven't replaced that those yet since they seemed to be okay. Could the coil also cause this type of issue? The plug wires could be original as they still have the plug numbering labels on them. What weird when it starts to die/cough/whatever its doing, if i turn it off and back on it'll run perfect again for a brief amount of time, then revert to same behavior as above. Any thoughts would be appreciated.
The coil, and the ignition controller (EZL) are possible sources of what you describe. So also is the coolant temp sensor. The W124/M103 package has been know to exhibit strange behavior as a result of a defective coolant sensor. Have you tried removing the fuel tank cap; possible venting issue?
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Old 04-02-2021, 12:55 PM
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Originally Posted by Frank Reiner View Post
The coil, and the ignition controller (EZL) are possible sources of what you describe. So also is the coolant temp sensor. The W124/M103 package has been know to exhibit strange behavior as a result of a defective coolant sensor. Have you tried removing the fuel tank cap; possible venting issue?
Thanks Frank. I have swapped out a good working EZL also(well so i think), but I have not messed around with the coil or coolant temp sensor yet. I'll try the gas cap and see if that helps with anything, I'm willing to try anything Coolant temp sensor would seem most plausible as once it gets the slightest amount of heat in it, it seems to start to do this weirdness. Still don't know why if i turn if off then right back on it'll run fine for a few then decide to go back to dying.
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Old 04-11-2021, 07:55 PM
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Alright I tried the pulling off the fuel cap while it started to do its typical behavior, no luck. I pulled two sensors from a parts 126(both in thermostat housing so assuming coolant sensors), not sure if they’re the same when it comes down to what the jetronic controller is looking for, but the plugs were the same. One had a singular pin plug and the other was a two plug type sensor, o rings on each connector. Tried each plug individually and separately, and it had no effect on how things have been behaving. I am really stumped. Next up would be the coil. I’ll see about getting one, and post my update!
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Old 04-15-2021, 07:58 PM
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New coil installed, which wont hurt to cross that off my list of things to replace before road worthiness. Absolutely no change from replacing the coil. I have no clue whats wrong. Same behavior, give throttle it'll rev up them and then die, rev up then die, all while my foot is on the throttle still. It will sit and idle very low around what appears to be 300 on the tach. Runs great for about 5 minutes, then it begins. Cycle the key and its good for about 15 seconds then back again.

I don't consider myself unqualified when it comes to mechanics, but it sure feels like it when I try to work on this car.
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Old 04-15-2021, 09:38 PM
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Originally Posted by ajschank View Post
New coil installed, which wont hurt to cross that off my list of things to replace before road worthiness. Absolutely no change from replacing the coil. I have no clue whats wrong. Same behavior, give throttle it'll rev up them and then die, rev up then die, all while my foot is on the throttle still. It will sit and idle very low around what appears to be 300 on the tach. Runs great for about 5 minutes, then it begins. Cycle the key and its good for about 15 seconds then back again.

I don't consider myself unqualified when it comes to mechanics, but it sure feels like it when I try to work on this car.
Consider this test: unplug the fuel pump relay, and jumper the sockets that correspond to pins 30 & 87 (usually 7 & 8). This will cause the pump(s) to run continuously. Start the engine, and see if the symptoms persist. If this effects a cure, the relay is indicted. If the symptoms are unchanged, there may be a pump problem.

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