Engine Problem

Hi

I have a problem with my engine.
No mechanic seems to be able to find a solution.
The car I have is C107/107023-1972. 3,5L V8. Motor 116.982. My problem is: at certain engine speeds the rpm drops drastically. i.e.: ca. @2700-2800 rpm at steady throttle it drops to ca. 1500. The same is repeated while driving. when the rpm drops I hear explosion sounds from the exhaust. Only when I depress the throttle further or decelerate by pulling my foot back the engine starts to rev up again and goes back to normal rpm starts to increase again. Sometimes I feel the same @ ard. 1700-1800 rpm and above 4000 rpm. The amboss( has been replaced with a new one from MB just recently. I have had the injectors cleaned last week (0280150034). out of 8 four are working with similar performance. two produce a higher output (+2-3 mg) and the other two are performing a bit less(-2-3mg). after each replacement all adjustments are made again. nothing changes. the last mechanic even tried different ecus one from 280 and 350 and man. Pres. sen. The fuel pump was recently replaced as well. I have continous pressure at all rpms 2.00 bar. the problem is there but may be softer. there is a cyclical repeat of this problem it reaches its climax at the high end of the torque. d-Jet parts match, pcb of ThPoSn. has been replaced with a new one. MPS was overhauled at Bosch(part bob). electrical wiring has been renewed incl. all connectors. I am a bit helpless. Would appreciate your contributions. You may reply in German. I can send a sound/video file
(Copy of a message you sent to Volker)

Hi,

that seems indeed a tricky issue. Even if you have replaced ECU and MPS with 350 compoents (280 does not fit!) and it still ist the same. Even new TPS. Did your mechanic measure both temperature sensors cold and warm? I guess so.

Waht are possible causes provided with all the things you have already swapped:

1. fuel pressure decreases at this speed. Connect a pressure gauge with a 2m hose and tape it wo wind shield. Does fuel pressure decrease at these speeds?
2. Sometimes we have seen loose trigger points. They should not moe at all when you try to do so.
3. If not we could still have a cabling issue

1- Fuel Pressure: On Friday I was at my mechanic. a manometer was attached to the fuel 'Ringleitung'. The injectors were not cleaned yet but the necessary adjustments were already made. at certain rpms we could hear exhaust explode and engine was stalling but pressure remained stable at all times. therefore I would exclude this possibility but another test can be performed.
After that my mechanic concluded that the problem should be with the injectors. This is why I spent the next day removing and cleaning injectors.
As the problem continued after cleaning I decided to consult the forum.
2- Loose trigger points: what do you mean by loose trigger points? I bought a brand new amboss by MB. Nothing has changed after the exchange. I had also adjusted my old one with your Gebisswerkzeug. I also check the positioning before installing the new one. If you mean the Zündverteiler' s shaft is moving loose and the injectors fail to receive command how can I check this? Why would the problem go away when the throttle position is changed?
the cables from the Zündverteiler to the spark plugs and the coil have been replaced with a new set.
3- for the combustion to happen we need spark, fuel and air. Considering we have the spark and the fuel (they are both operated by the ZV) what about the air. Could reduction in the amount of air cause such a problem? i.e. when the Zusatzluftschieber hose is pressed the engine rpm drops?
4- What should I check about cabling? I have all the wiring diagrams from ecu to sensors
5- would it make sense to gradually remove sensors like the MPS, Ansauglufttemperaturfühler, TPS, etc.
I would like to repeat again that engine is not stopping or trying to shut down it slows down asIf it is being choked.

Hi,

I would like to greet you with your name, but that would need you to sign with your first name.

Exhaust explosions normally mean unburned fuel in exhaust. That typically happens with improper igntion voltage or timing. I would recommend you to check igntion timing at idle, 1500 and 3000 rpm comparing to workshop manual values.

Fuel pressure could change if fule lines are somehow blocked. That is why I asked to check at all rpms.

Even brand new trigger contacts can be misaligned or be loose. I recently had two loose screws under the soldering points.

Missing air cannot be the problem as you deliver it via throttle. AAV can increase rpm but not drop it if it is closed.

If you remove MPS it will stall the engine. ECU needs MPS to operate.

Hi

My name is Juan. Thank you for the suggestions.
I will check and will revert to you

Rgds
Juan

Hi Juan,
be very careful... backfiring can kill the diaphragm of the MPS.

You said the TPS was revised with a new PCB. Maybe the contacts for acceleration (at the rotating unit) or the PCB itself / the sleeve contacts were greased? Don't grease these parts, just clean them. They won't give any contact if greased because the electric current is very low.

Did you check the clearance of the engine valves, inspected the engine camshafts? I don't know, maybe your engine has "Hydrostößel" (don't know the word, 'automatic valve clearance setting')? Can get defective, too.

But please begin by checking the ignition as Volker wrote before.

You asked if it's possible to remove the sensors.

You could replace the two temp sensors with fix resistors, both 200 Ohm, min. 1/4 Watt for testing.
This would simulate normal air temperature and warm engine.

Removing the TPS electrically would mean missing idle signal / no stable idle, going up and down.
No impulses when accelerating = problems in accelerating will be the result.
But: No problems should occur in constant driving conditions, ignition is regulated by the MPS signal then.

Regards
Norbert