Long Warm Start

[owenj99]

Hi guys,

I have been scouring this forum trying to find some input to my problem and have found similar issues, but the remedies to them have not fixed my problems....

On my 1992 500E, I have a long warm start issue that occurs immediately after turning the car off. What I mean by that is it doesn't matter if the car sits for 5 seconds or 30 minutes, when warm, it still has a long start. I thought this could be the fuel pressure regulator, which I replaced a few months ago, but was worried it could be defective. Replaced again and no difference. Additionally, both fuel pumps (with new check valves) and fuel filter were replaced shortly before the long start occurred. The LH module has also been replaced to fix a long cold start issue (fuel pumps wouldn't prime during cold start, causing a long and rough start. Module fixed the cold start issue). I initially though maybe one of the pumps was bad, so I replaced both to still have the same issue... swapped with the LH module for my 94 E500 and it would start right up when cold.

Thinking maybe the used module I got fixed the cold start issue but created the warm start issue, I swapped the old module back into the car (never had this issue with that module installed, only the cold start issue) and the long warm start still occured. Additionally, I disconnected the MAF sensor and the long start still occured, but was somewhat intermittent. It started regularly only once with a long start every time otherwise, but then when I reconnected the MAF it started normally again as well, therefore I think it may be unrelated to the MAF itswlr and could be just a fluke event where it actually worked as it should.

I have attached a link to my youtube video showing the long start. This is after idling the car in the driveway for roughly 10-15 minutes with about 80° temps outside. It's an odd long start because it doesn't stumble or run rough when the car does finally restart. It like a switch is flipped and then the car fires up after maybe 3 or 4 seconds and runs perfectly.

Thanks in advance for the advice!

Owen


Long start video link -

 

[gsxr]

Owen, are there any recurring fault codes on any module?

 

[owenj99]

Hey Dave,

I'll double check today. I haven't driven the car much because I'm worried it's not going to start while I'm out, so hopefully the codes have had time to store.

I'll update this afternoon.

 

[owenj99]

So I just read the current stored codes in the car. Planning on taking out for some errands shortly so I will recheck the codes after that. I've attached a screenshot of the codes.

I'd like to note that I suspect the MAF sensor code and LH codes are related to me trying to diagnose as I haven't cleared the codes since having them unplugged...

 

[gsxr]

I assume you cleared all those codes after reading them, correct? Let's see which (if any) return after some long cranking times. I assume the LH ECT codes were from the connector being unplugged temporarily while the ignition was on, but it wouldn't hurt to check the ECT with an ohm meter and see if the values are normal.

 

[owenj99]

Yes, all codes are cleared now. If I remember correctly all 3 temp sensors were recently replaced on the car (less than a year ago, but that also could have been on the '94... I'll check the condition of them as well.) Could an ECT issue cause something like this?

I recheck the codes after running my errands today.

 

[gsxr]

ECT can cause starting issues if the reading is too cold or too warm. Use the PDF below to check the ECT compared to the dash temp gauge, assuming you don't have a scanner to view live data. Alternately you can swap sensors from another car if you have two M119's with LH.

https://www.w124performance.com/docs/mb/M119/coolant_temp_sensor_4-pin.pdf

 

[owenj99]

Hey dave,

No live data unfortunately. Any insight on the best way to test the sensors? I saw the data values on the PDF to check for but am unsure about the best way to test them?

Took the car out for errands today. 3 starts in total so hopefully that's enough for the computer to store some codes. Waiting for the engine bay to cool down and the garage to cool back down then I'll read the codes.

Not to throw a big wrench in the thinking, but I've also had an intermittent no A/C event. This is another strange one, but after hearing the temp sensor issues, I think they could be related. When the engine is cold the A/C works perfectly. If you drive the car normally (no kick down, heavy load, e.t.c. ) the A/C will keep working for a while. Not sure how long, but normally until I shut the engine off again. Fans also function as intended. Once the engine is up to temp, after you kick down the A/C will shut off and the fans stay on high speed like the engine temp is too high. The only way to get the A/C to work again is to turn the car off and on again (50/50 that it works afterwards or not), or have the car under medium to heavy load or kick down and the AC will run the whole time the car is under load. Fans also seems to get confused and rapidly switch between low speed and high speed.

Temp sensor issue? Idk. I'll report back with the codes in a bit. I've linked two videos, one of the fans running weird, and one of the low A/C pressure changing during throttle input. I'd also like to note that the A/C has been charged properly by a mechanic, but my good A/c gauges are no where to be found at the moment. Lol

Fans -

A/C pressure Gauge -

Thanks in advance

 

[owenj99]

Welp - you were right Dave!

4 prong ECT sensor appears to have done it. Swapped it with the sensor from the '94 E500 and now the fans are working normally and warm start is normal. The only two codes that returned after driving today were the same as the previous two for the ECT sensor.

I swapped the "bad" sensor from the '92 into the '94 just to confirm, and now both cars appear to be working just fine. Bonus special! Ha! Maybe loose connector on the '92? Idk but I'm happy with it either way.

All 3 new sensors for both cars on order just to prevent any other future issues.

Thanks for all the input and the help diagnosing!

 

[gsxr]

You appear to have at least 2 separate issues, maybe more.

1) Check the temp sensors with an ohm meter. PDF's are on my site for all three sensors, check at ambient temp (~20C) and at normal operating temp (80-90C), the sensors should all read close to spec at both temps. If any are way off, replace with OE only - I believe all aftermarket are now junk regardless of brand. (?)

2) The high speed electric fans are triggered only by engine temp based on the TWO-pin sensor, controlled through the HVAC pushbutton unit. It could either be a sensor problem, or a PBU issue. If you have a "Cool Harness" installed, remove it. Normal operation is the fans turn on at 107C and turn off at something like 98C (forget the exact temp). Also check the temp sensor with an ohm meter, note the resistance values are completely different between each sensor (1-, 2-, and 3-pin).

3) I assume your compressor is off/disengaged when the refrigerant gauge is in the red zone, and is engaging when the needle drops rapidly to the green zone. This could be a number of things: PBU control, faulty BM/GM, or the pressure safety switch (not likely but you can bypass it for testing).

4) The fans & AC are probably not related to the starting issue, but could be, maybe.


I'm suspicious of your PBU, do you have another 124 that you can swap it with?

EDIT - you were posting at the same time I was, lol. The 4-pin sensor is only for the engine computers (could affect starting but not AC or fans), the 2-pin sensor is only for the HVAC system (will affect fans & AC, but not starting). The 1-pin sensor is only for the dash gauge. How are the fans & AC compressor behaving?

 

[owenj99]

Haha whoops!

For #1 - I will go through and check the ohm of the new sensors once they arrive and make sure that all the sensors align. Maybe the 2 prong sensor was also loose? I had to remove the connectors for all three sensors as I didnt have a socket that would fit over the four prong sensor. I only swapped the 4 prong sensors between the two cars. The others weren't swapped.

No cool harness on the fans. But with the fan issue and the fans always wanting to run on high, the temp has not get above ~100°C even with exterior temps in like mid 90°F. So I havent been able to confirm the A/C pressure switch but that was my initial thought as well.

To confirm - yes, when the pressure would spike on the gauge, the compressor was disengaging. From what I can tell, since the 4 prong was swapped, all engine, A/C and fans seem to be operating as intended. The previous issue where the compressor would only cycle when the engine was under load is no longer occurring and I can now kick down again without the A/C cutting out afterwards.

I will do some more driving with the car tomorrow to confirm, but during my two 10-15 minute test drives (which was enough to cause the previously mentioned issues) none of the issues occured. I'll keep you posted with how it everything goes but hopefully it was just a sensor issue. If not I'll dive further into the fan issue. One of the bearings is starting to squeak in the fan anyways, so its about to be time to pull them out and replace them while the car isnt being used as much this winter.

I'll update tomorrow with any new findings.

 

[gsxr]

One more thing... if the AC stops after a WOT blast, and won't turn back on until you restart the car, that is 99.44% likely to be a slipping belt. How old is your manual belt tensioner on the '92?

The electric fans should never turn on/off rapidly as in your video, that is really bizarre. If that happens again with new/different 2-pin sensors, try swapping the HVAC PBU between cars.

 

[owenj99]

The tensioner hasn't been changed during my ownership, but a new 8 rib belt was installed less than a year ago. I can recheck the tension but it was installed to factory tension maybe 1500 or 2k miles ago.

What is the PBU? I don't think I have heard of that unit/module before...

 

[a777fan]

PBU -push button unit. The HVAC control unit with the buttons and the ‘Price is Right’ showcase showdown temp wheel in the center console.

 

[gsxr]

The tensioner hasn't been changed during my ownership, but a new 8 rib belt was installed less than a year ago. I can recheck the tension but it was installed to factory tension maybe 1500 or 2k miles ago.

The tensioner is just a rubber bushing that is forced to twist against its will. They are not known for a long lifespan and can fail without much warning. The proper test is to remove & check with a bench vise as shown in the FSM, but you can get an idea by pulling upward on the belt by hand. Compare to your other car. When the bushing fails, the plastic pointer/indicator may show correct tension but the belt will be too loose.

 

[owenj99]

Okay, So after about 150-ish miles over the weekend and today, I have some new discoveries... Few good and many unfortunate

1. Warm start is still working as intended! Woo Hoo!

2. A/C works under all driving conditions! Extra Woo Hoo! considering it was hot as hell today.

3. Fans still do weird stuff. high and low switch as in the video.

4. checked the wiring to the 2 prong sensor and got some super extra, extremely terrible, worse news possible kind of news...

5. I need a new upper wiring harness Wiring to the two prong sensor behind the rubberized wire liner thing has just disintegrated. couldn't even tell which color insulation was on the wires....

So, my next question is what is the correct wiring harness part number for a 1992 500E? I've seen a few numbers floating through the forum. One was a 1244402006 and another being 1245439526. I have attached a picture of the VIN and production date to hopefully help clarify because I am completely confused on which is which. Both of the numbers show currently available from Mercedes of Annapolis parts center. I know the 1244401906 is for the 400E/E420, but what is the difference sensor wise? Connectors look the same on all three harnesses...

Thanks,

Owen

 

[gsxr]

Actually that is good news! The old upper harness is likely causing the fans to wig out, and also could be causing some of the other issues (starting, etc).

1992 500E should use 124-543-95-26 per the EPC below. Around $450 for a new one which is a bargain (I'm sure NLA is coming soon), only takes an hour or two to swap out.

As to the differences between the harnesses, it seems there was some minor internal changes and possibly different insulation colors, but the early/late should be interchangeable. Details are in the attachments at this thread. The 034 harnesses are electrically similar but have different routing through the firewall and generally shouldn't be interchanged with 036.

 

[owenj99]

Awesome! Thanks again for the help, you're the man!

I thought I was going to be in for a really expensive treat as the last time I looked for a harness they were NLA and the ones you could find were $$$$. I'm also happily surprised to hear that it's only an hour or two for labor! I was mentally preparing to pull half the engine bay apart.

Haven't ordered the harness yet, but once I get around to getting that done, along with my laundry list of jobs to do on the other cars, I will update. I'm really hoping this sorts out all of the little gremlins I have been battling

 

[owenj99]

Hey guys,

So I again have some good news and some bad news haha!

Good news - New harness is installed as well as all three new temp sensors. Car idles better and a mild hesitation that it had at idle is now gone. Also - still no warm start issues! Whoo hoo! The harness was absolutely trashed. Can't believe this thing even ran, honestly...

Bad news - A/C fans started out still acting sporadically, running normally sometimes and switching to high speed randomly like in my previous video. A/C was also still intermittent, working most of the time and switching off and on based on throttle position. Kick down when it was off and it may come back on and stay on, kick down again later on and the A/C may stay off, or come back on. Couldn't really get an exact pattern after the harness replacement.

As of today, moved the car out of the garage to wash it and before I even started the engine the fans were on high. No A/C either. Washed the car and took it in town for some errands, and no A/C the whole trip, including multiple key cycles from various stops, kick downs, etc. Fans were constantly on high throughout all trips. As soon as the key was turned in the ignition the fans were on max. I haven't added or removed any Freon from the system.

Any ideas? Thanks in advance!

Owen

 

[gsxr]

Owen, as mentioned previously in post #12, the high speed fan is turned on/off by the HVAC pushbutton unit in the center console. Try swapping that out next. This may also help with the A/C issue, if there are no fault codes on the BM/GM.

 

[owenj99]

Sounds good, I will try swapping with the '94 later this week and see what happens.

Thanks!

 

[owenj99]

Update - Swapped the HVAC PBU with the known working one from the '94 and no change. Fans still turn on immediately upon cycling the key and no A/C with engine running. Compressor doesn't even try to cycle upon start and fans still run at max speed. Placed the questionable unit in the '94 and everything still functioned as intended.

Checked the codes (had to remove the fan relay so they didn't run the whole time I was checking and kill the battery ) and found a few codes I believe to be from the harness issues such as a single temp sensor code and an IAT sensor code from when I had that disconnected. The A/C clutch jammed / V-belt broken code was the only code in the Base Module.

I have attached a screenshot of all the codes that were stored. Any other ideas?

Thanks!
Owen