Hello all.

New to the forum here. I got my new to me 240 sedan a few months ago and I have used this forum extensively to fix quite a few things. This forum has been so helpful!
I am struggling with an issue that seems to be erratic and I am at wits end on how to fix it.

Problems:
1. When I start the car for the first time, it will start up just fine and idle normally for a few seconds. Then it will stumble really bad as if it were going to stall, but never stalls. Giving it some gas helps. Once it starts warming up and the temp gauge starts moving, it starts idling normally. This behavior is consistent and happens every time.
2. If I try to take off from a stop sign or stop light, there's usually hesitation after which it runs normally. This is not consistent. Sometimes it runs with no hesitation, other times it bogs down really bad and sometimes it's just a mild hesitation. This happens irrespective of fuel level. It is also independent of engine operating temperature.

Symptoms: Outside of what I have mentioned above, the gas mileage has dropped. I used to usually get 22-23mpg. Now I'm down to 17-18mpg. I also think the car is running lean. My understanding is that running rich would mean smelling raw fuel - which is not the case here.

Things I have done based on old threads I read here:
1. Swapped plugs and distributor to Bosch and wires to Bougicord. Plug 1 was ever so tiny bit wet.
2. Air filter is clean.
3. Cleaned out the flame trap. Also cleaned up the throttle body flap and the IAC. The IAC is gunk free and I can move the flap inside.
4. Swapped out the -016 AMM with another used Bosch one. No difference
5. Did a compression test. All numbers are even and very close to each other.
6. Verified that the in tank fuel pump primes (so I assume it runs correctly). Found an Airtex pump box in the trunk under the carpeting from 2016.
7. Fuel pressure regulator smelt like raw fuel. So I replaced it with a new Bosch FPR. New regulator doesn't smell like fuel. Anything else I can do verify this is ok outside of buying a pressure gauge kit?
8. Verified that all injectors are clicking with mechanics stethescope. Also verified that all 4 injectors are getting battery voltage at the connectors.
9. Verified that the O2 sensor bounces around 0.5V when the car is revved up and then idled back (not sure if this means much)
10. I was trying to check the ECT. When the car was nice and warm, I couldnt get a steady reading on my multimeter. The readings I got were bouncing between 0.5 and 200/210 at pins 5 and 13 on the ECU connector. The car cooled down for an hour - I was able to get a steady reading of 57 in the 2k Ohms setting (so 570 Ohms? Sorry, my electrical skills need work). The engine was warm to touch but hot by any means. I plan to check the number again either tonight or tomorrow morning.
Interestingly enough, I am unable to get any readings (DMM says 1 always) on pins 2 and 11 at the ICU connector.

Fuel filter seems to have been replaced not too long ago based on it looking relatively new when I crawled under the car.

At this point, I'm lost. I really do not know how to proceed. No CEL and the little probe on the LHS shock tower didn't register any codes. 1-1-1 in sockets 2 and 6.
Outside of suspecting the ECT I am completely stumped by this car. If someone can point me what else I can do to fix this, I would appreciate it!

Thank you.

 

I had a very similar case with a '93 240 several months back, many said 'computer', (always a favorite when stumped). It was the AMM. I know you say you changed yours with a used one, I'd try another.

 

Thank you for your response! The junkyards around here are closed due to the ongoing coronavirus, so it might be a little while before I can get another AMM.
Would trying some "MAF sensor cleaner" be of any use? Would unplugging the car and driving it around for a short distance help rule out the hesitation issue being related to the AMM?

 

easy way to see if you are running rich/lean is the plug color. running rich usually leaves carbon or oily looking plugs. Running lean will leave light tan or light gray plugs with some possible pitting of the electrode.

 

^Thanks! I did not know that. They were definitely light tan - so running lean. Plug from cylinder 1 also looked wet. Could it be a leaky injector?
I unplugged the AMM and it gave me 1-2-1 code and ran rather roughly with RPM bouncing around 600. However, they were consistent for the couple of minutes I had the car idling.
I cleaned the AMM with the circuitry facing upwards and plugged it back in. That interestingly made the car run even worse.
So switched back to my original AMM. The reason why I had switched the old AMM out was because it would run fine cold, but would cause some stuttering and hiccuping when the engine is warmed up and I'm driving around.

Which makes me think Lev is probably up to something. Doesn't seem like there any B230F cars around me in junkyards within a 100 miles range. Will eBay knock offs work? I'm afraid that the used Bosch ones off eBay will give me the same problems.
A new genuine one on iPD costs $260!!

 

I wanted to post an update. At this point, I got cleaned out my original AMM with the MAF cleaner from the parts store. Did it twice very thoroughly with the circuitry up to avoid any run-offs into it.
As of now, I am about 300 miles in and the car has been running great. Problem #1 from my original post is completely gone. I will continue putting the miles and see if problem #2 is also solved (fingers crossed)

 

I had a similar issue with stumbling. It would be quite scary when you press the gas and nothing happens. Anyhow I was getting a code for TPS. The LH 2.4 TPS acts as a on/off switch. It turned out that the throttle body wasn’t fully closing. Just make sure when the throttle returns to “rest”, it is actually closing. You would know because you will hear a “click”. The procedure is pretty well described in the service manual. I had to adjust the tension on the throttle cable to ensure that the throttle was closing. I didn’t mess with the TPS sensor at all. It stopped stumbling and the code went away.

 

also a throttle thats gummed up with deposits can not close completely and not engage the TPS aka idle switch.

 

What is an AMM?

 

another name for the MAF. Air Mass Meter, aka Mass Air Flow ... LH injection (actually LH-Jetronic), the LH stands for Luftmasse-Hitzdraht, which is literraly Air-Mass - Hot Wire. the MAF/AMM for LH has a heated wire in the air path, the amount of air flowing past this wire cools it, and its resistance varies with the temperature, so more air == cooler wire == more (or less?) resistance (I forget which way it goes). this is used by the ECU to decide how much fuel to inject each cycle.

 

This is great advice. I went out and checked the throttle body. The flap does close properly. I had cleaned up the flap when I got the vehicle because it had so much build up that originally it wouldn’t close.

I did verify that the TPS clicks when you move the throttle even a tiny bit.

 

I cleaned up the throttle body and idle control valve (?) when I did some maintenance on purchasing the vehicle. Both were nasty. They are clean now.

 

any air leaks in the intake pathway downstream from the MAF/AMM will cause stumbling idles and stalling at low RPMs.

 

I found a couple of small nicks in the accordion hose that I have “mended”. Not sure how permanent the fix will be. But the surging has definitely gone away for now.