1995 E320 W124 M104 idle / stumble woes
 

Hi all you Bosch HFM fans. I used to sit smugly as my beast purred smooth and faithful. Now that she's rolled over 159K, though, she has begun acting up.

Symptoms:
1. Oscillating idle (not all the time), sometimes leading to stall.
2. Occasional stumble at speed.
3. Exhaust smell. Sometimes sulfury, sometimes just rich.

What it is not:
1. EGR valve or pipe. Checked w/ vacuum pump, and cleaned pipe.
2. Fuel pressure regulator. Replaced as a cheap attempt after reading an MB recommendation.
3. Oxygen sensor. Replaced. Old one was black & sooty, leading me to suspect rich running condition. After replacing, engine seems more hesitant on acceleration, as if too lean.
4. Vacuum leaks. One repaired, no others visible.
5. Probably not harness. No visible cracking, though I haven't cut into the main line sheath.

Codes: I have had at various times 5, 6, 19, but they come on at stalling, so I'm not sure if they are the cause or the effect.

Since I suspect a mixture problem, I'm leaning towards the MAF, though presumably injectors or throttle bnit could maybe be suspect. I know zamar___ replaced his ECU when he had similar troubles, but I need to rule everything else out before parting with $1500.

I would like to validate the HFM air flow sensor before throwing $300 at it. Are there any good tests?

All ideas and tips appreciated.
Thanks,
Tom

Check if the OVP has been updated and change the plug connectors under each coil...

Thanks, Arthur, I'll check.
Now for the dumb question: where is the OVP on mine, driver's side or passenger?
There is one black box next to the fuse box which has 2 fuses of its own. They are OK.


Tom

Behind panel, rear of battery..
If it is the new style , leave it be..

The plug connectors are under each coil and I only recommend changing them as an easy diagnostic procedure as they are cheap and are a common wear item that will cause your problem

I think your codes were from earlier causes, ie vac leak/reg possibles
You will know more w/ connectors and relay check..

Check the codes from Pin 8 and 14. In my case, the air/fuel mixture side of the ECU had gone into a fail safe mode and replacing the ECU fixed the problem.

I did not have any symptoms, except for the check engine light and maybe a slightly higher fuel consumption.

The codes I had were:

DM Codes 3,4,19
Pin 8 Code 13

In your case, the problem appears to be related to the Throttle Control, PIN 14 should give you some information.

I would eliminate the fuel pump relay, OVP, plug/coil connectors, MAS as the causes, before replacing the ECU.

The usual method of testing the MAS and ECU is to replace them with known good units. That's how I tested them, and still have the old MAS (good) and the old ECU (causes the check engine light to come on but does not effect driveability).

The OVP relay is a tall, silver-colored box. Replacing its 15A fuse had no effect. Does the relay just pull out, or is held in place by a screw?


Also, the symptoms are not totally random. The car runs fine at speed. The stumbling / stalling occurs maybe 10-15 secs. after slowing, as though there is a problem adjusting mixture to the new engine speed.

Again, CE light came on only after full stall, but I haven't had time to read the codes - suspect 5 and 19 again.

There is a screw on the side.
Look at the part # to see if it is orig . [ more likely not]
The reason for changing the plug/coil connectors before proceeding with further test is that as soon as the ecu recieves an ignition skip, it cuts the elect sig to the injectors... so the stumble is most noted as low rpms [ as you state] .
When it does mis-fire , it also effects 2 plugs rather than one as this is waste spark system and 2 plugs fire in series.
Codes on other modules will be your next step, but you want to assure ignition first without spending big $$$ . The connectors are known faults for you problem, and changing them is easier than scoping them out..

Arthur,
The under coil connectors (A) are clear, but do you mean a whole wire set for the other plugs?

Thanks,
Tom

NO
Just the connector/resistors under each coil [ 3 ]
They break down from heat as they are trapped under the coils..
The spring on the connector looses temper [ it is a friction connection] and fails , whereas the other 3 plugs have snap on connectors , so they should be OK..

While there , I always check the plugs cuz they will be carboned from mis-fire.
You prolly already know to use only Bosch F8DC4 on that engine [ again, cheap to just change them..]
I seee so many guys change ECUs and throttle acuators, MAFs , etc , and wind up solving their 104 problems with some connectors and plugs..
So, I start at that end first..........along with the newer OVP.

OVP part number is 000 540 67 45. IS that the new and "improved" one?

Plug connectors scheduled for tomorrow.

The ASR light came on briefly also, but given the heavy rain, I suspect it may be due to that???

That is the new part
Old is 540-52-45

Somebody once posted that the cowling in front of the windshield channels water and dumps it onto the OVP, exacerbating the problem.

OK plug connectors on the coils replaced - stumbling at speed corrected, but unstable idle remains.

The old parts were in fairly good shape, and all plugs had a gap of ca .04". The electrodes were whitish, and all plugs were dry.

I read the code from the last stall, and had only a 10 ("Voltage too high at HFM").

I now think that I had a few things needing attention, but the whole HFM circuit was able to compensate until this mystery culprit failed a few weeks ago. ( I also fixed a vacuum leak, replaced O2 sensor and fuel pressure regulator). Up until the O2 sensor swap, I was getting 5's, 6,s, occasional 10, and 19.

Next diagnostic recommendation?

Thanks,
Tom

Now would be a good time to go to pin #8 and see what has come up.
You can find a $5 tool in archives or email me..

Wilco. After a weekend trip to DC (in the Ford).

Tom