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Die Bosch D-Jetronic war 1967 die erste Großserien elektronische Einspritzung der Welt. - Bosch's D-Jetronic was the first mass-production electronic fuel injection.

Engine Shaking at Idle

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2 Jahre 3 Monate her - 2 Jahre 3 Monate her #17280 von andya
Engine Shaking at Idle wurde erstellt von andya
Hi, I am having a problem with my 1974 450SL Mercedes, which is a D-Jetronic V8. Any suggestions on what I can check next? Today I will check the flow of the injectors into small jars and also resistances at the ECU connector, but I am running out of ideas.

Thanks! Andy

Problem

When I turn the car on the cold engine is shaking enough to shake the car. As it warms up the revs drop and the shaking becomes worse and the engine seems quite unhappy.

General Observations

G1. Acceleration and power appear to be OK.

G2. Shaking is only at idle.

G3. The revs are stable. At cold they are at 1,200 to 1,300 and they gradually drop to 700.

G4. Some smoke out of the tailpipe when first turned on but it quickly dissipates and it is hard to tell the color. Car has always done that if not regularly driven.

G5. Engine sounds OK and revving is smooth.

Air

A1. I have rebuilt the AAV and tested it with water at various temperatures. I think the fact that the revs drop as it warms up and I don't get surging indicate to me that it is working.

A2. Vacuum at the manifold at warm idle is 15inHg. I am at 2,500 ft ASL.

A3. I have set the position of the throttle position sensor with a feeler gauge so that the first three degrees of pedal press don't open the throttle plate per the factory service manual. Visually checked the throttle plate is closed at idle.

A4. The throttle body has been cleaned including the small hole for the vacuum takeoff.

Ignition

I1. I replaced the points with Pertronix a couple of years ago and I have never had a problem with it. I put my scope on the output of the Pertronix and it shows a square waveform at a stable frequency as expected. There are glitches above 20V according to my scope but they are not continuous.

I2. Switchgear is not bypassed.

I3. Coil is original Bosch and I have tried a spare Bosch coil (same part number) and no improvement.

I4. Rotor, cap and wires were all replaced a couple of years ago. I've tested with brand new spark plugs gapped to 0.032" does not give an improvement.

I5. I don't know if the spark is consistent because I haven't been able to find a spark tester that will fit with my wires and with plugs with the push on caps removed (which is how I use them).

I6. I have put in a brand new reman alternator. No improvement.

I7. Battery voltage is fine - all cells are functional - 12.6V output. Battery is only a couple of years old.

I8. I have not changed the voltage regulator.

I9. I have double-checked the ignition wires are all correctly located and fully connected to the spark plugs and cap.

I10. Timing at idle is 8 deg BTDC. I found this gives the best acceleration and I have not changed it in two years. I've tried 5 deg ATDC to 30 deg BTDC and no change in the shaking.

I11. Distributor weights and vacuum plate have been cleaned and lubricated.

I12. The vacuum pod on the distributor appears to have a leak however there is no leak in the switchover valve and AC is turned off during these tests.

Injection

J1. Fuel pump is outputting one litre in 30 seconds. I bypassed the filter and it still outputs one litre in 30 seconds. Voltage to the pump has a 1.4V PTP ripple on it - not sure if this is normal or not. Voltage was measured at the fuel pump relay.

J2. Fuel pressure is 30psi. During injection there is a high frequency variation of the fuel pressure from approx. 26 to 31 psi. I have found a 350SL owner who has the same thing but with no engine shaking.

J3. If I force the engine into an over-run state by pushing on the throttle linkage and then releasing it, while the revs drop the fuel pressure oscillations stop and the engine shaking stops. I know that injection has stopped in this state, so clearly fuel pressure oscillations are caused by injection.

J4. I have rebuilt the trigger points with new blocks. Dwells were measured at (degrees): 136, 134, 121 and 132.

J5. Throttle Position Switch has been cleaned and tested.

J6. It appears to be running very rich. I don't have a CO or AFR meter at the moment. The knob on the ECU is six clicks from fully counter-clockwise so I take that to be six clicks from fully lean. Setting it fully counter-clockwise does not stall the engine or make it any worse running than it currently is.

J7. The MPS I am testing with is not original to the car but it has it's plastic cover intact.

J8. All my testing is at cold idle and occasionally warm idle and all of the plugs are carbon fouled within a few days of doing this.

J9. Soot coming out of the tailpipe.

J10. I have put Techron and premium fuel (91 octane) into the tank and drove the car up and down the freeway at 75MPH and no improvement.

J11. The engine wiring harness is brand new.

J12. I have cleaned the metal grounding point and eyelets for the wiring harness near cyl #4.

J13. Injectors have been cleaned in an ultrasonic cleaner with Techron. They received new seals and pintle caps two years ago.

J14. I have tested the T2 coolant temp sensor at various temperatures and it matches the specifications.

J15. Fuel pressure leak down test was successful - meets specs in factory service manual.

J16. I put my stethoscope on each injector in turn and they they were all clicking away without pauses and seemingly at the same rate.

J17. Fuel pressure on return line between damper and tank is zero psi.

J18. I got the engine hot and connected my scope to the tach signal. It fluctuated around 40 - 41 Hz. I then unplugged each injector, measured the new frequency and plugged it back in. Results in cylinder order (Hz): 37.8, 37, 37.2, 38, 38.4, 36.2, 37, 39.

J19. Injectors each output 7cc3 per minute during cranking with 30psi (2 bar) of fuel rail pressure

Mechanical

M1. Compression of each cylinder in order is (psi): 116, 127, 124, 118, 122, 117, 112, 112. Total compression range 1.02 ATM..

M2. There may be oil in cylinders #3 and #4 - I have to investigate this further.

M3. Valve lash has not been checked.
Letzte Änderung: 2 Jahre 3 Monate her von andya.

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2 Jahre 3 Monate her #17281 von Dr-DJet
Dr-DJet antwortete auf Engine Shaking at Idle
Hi Andy,

oops so much information. Well 2 things as a quick comment:
  1. Did you verify ignition timing at idle, 1500, 3000 rpm? Plus vacuum retard?
  2. How is exhaust CO and HC in warm idle?
It possible please use SI measures when specifying pressures, temperatures, volumes. Either your ignition timing seems wrong or you have a bad fuel mixture. It is normal that your car would tend to run better in cold and higher revs and start to go worse as soon as it gets to warm idle.

Viele Schraubergrüße - best regards, Dr-DJet Volker
Alles für den Mercedes-Benz R/C 107 und W116 in der SLpedia Sternzeit 107

Workshop Hzg/Klima 13.4 (ER), D-Jetronic 8.6. (F) & 20.7. (ER), K-Jetronic 29.6. (ER) & 31.8. (F)

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2 Jahre 3 Monate her #17282 von andya
andya antwortete auf Engine Shaking at Idle
1. Not yet, only idle and I set it there because that gave best performance, but changing it at idle didn't help with the shaking at idle.

2. I don't have a meter. I don't want to modify my vehicle by cutting holes in the exhaust - do you have a recommendation?

However even if I set the ECU knob to the leanest setting it doesn't help. What other adjustment options do I have?

Thanks! Andy

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2 Jahre 3 Monate her #17283 von Dr-DJet
Dr-DJet antwortete auf Engine Shaking at Idle
Hi Andy,

measuring would be best: Use a digital ignition test gun like Equus 5568. Espeicially after you disassembled you distributor. Or even better an engine and 4-gas exhaust tester. Bosch, Sun ... does not matter. They can analyze ignition primary and secondary voltage via oscilloscope plus measure CO and HC.

Viele Schraubergrüße - best regards, Dr-DJet Volker
Alles für den Mercedes-Benz R/C 107 und W116 in der SLpedia Sternzeit 107

Workshop Hzg/Klima 13.4 (ER), D-Jetronic 8.6. (F) & 20.7. (ER), K-Jetronic 29.6. (ER) & 31.8. (F)

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2 Jahre 3 Monate her - 2 Jahre 3 Monate her #17284 von andya
andya antwortete auf Engine Shaking at Idle
Thanks I will look into that.

Meanwhile is the voltage ripple to the fuel pump OK?

Are the fuel pressure fluctuations during injection OK?

I would like to at least rule out the electrical system, the fuel pressure regulator and the fuel damper.

Thanks! Andy
Letzte Änderung: 2 Jahre 3 Monate her von andya.

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2 Jahre 3 Monate her #17285 von Dr-DJet
Dr-DJet antwortete auf Engine Shaking at Idle
Hi Andy,

oscillations in fuel pressure come from opening and closing injectors. I cannot comment on your oscillations as I neither know psi nor your gauge. But it is no surprise that you have no oscillations in over-run. Injectors are switched off in over-run.

Viele Schraubergrüße - best regards, Dr-DJet Volker
Alles für den Mercedes-Benz R/C 107 und W116 in der SLpedia Sternzeit 107

Workshop Hzg/Klima 13.4 (ER), D-Jetronic 8.6. (F) & 20.7. (ER), K-Jetronic 29.6. (ER) & 31.8. (F)

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