1972 280SE 4.5 misfire - Page 2 - PeachParts Mercedes-Benz Forum

Tomguy · #16 06-07-2009, 10:52 AM

Quote:

Originally Posted by Hugin

Hi

Have checked injectors, they all click alike.
Have checked all plugs, they all fire normaly.
Have checked fuel pressure, it's 2,1 bar=appx 30 PSI.

A local wizzard suggested the MAP-sensor, he had a similar fault on his a while ago.

A lot of people have spare 4.5 maps - I may have one somewhere - but I don't see you checked the trigger points. I would make 100% sure to check them first!

Hugin · #17 06-07-2009, 11:27 AM

Hi

I have made arrangements to buy a MAP sensor, and if it isn't the sensor, it's a nice thing to have anyway.

As for the triggerpoints, I can say that all the injectors click alike.
Have just purchased the service manual, comming by mail from US in a week.

Meenwhile, can you explain to me how I check the trigger points?
Does it involve a lot of dismounting?

When I exchange the MAP sensor, is there a need to make adjustments to make it fit my car, or are they all calibrated the same way from the factory?

Tom

bwostosh · #18 06-07-2009, 01:47 PM

MAP (Manifold Air Pressure) sensor will adjust the over-all fuel mixture rich or lean operating on the vacuum differential between atmospheric pressure and engine vacuum. Example, adjustment will compensate for low vacuum from worn engine. But do this after everything else is perfected. It can be done on a road test. Last one I did was a 1,500mile road test / adjustment 450SL.

Tomguy · #19 06-07-2009, 07:38 PM

The MAP sensor, if you switch it out, will require re-adjustment to your car and probably won't solve the issue if you didn't first make 100% sure you have no other vacuum leaks (especially the hose to the MAP sensor), and then try adjusting your current one. The trigger points will require that the distributor be removed to access them. You will need to re-time the engine, of course, on reinstalling it. The whole job doesn't even take me 15 minutes anymore!

Hugin · #20 06-08-2009, 01:52 AM

Quote:

Originally Posted by Tomguy

The MAP sensor, if you switch it out, will require re-adjustment to your car and probably won't solve the issue if you didn't first make 100% sure you have no other vacuum leaks (especially the hose to the MAP sensor), and then try adjusting your current one. The trigger points will require that the distributor be removed to access them. You will need to re-time the engine, of course, on reinstalling it. The whole job doesn't even take me 15 minutes anymore!

Ok Tom, thanks again for your help.

I will look for vacuum leaks, but as this problem appeared suddenly, and the hoses looks good, I have my doubts here.

Again, if I understand you right, you remove the whole distributor, and not just parts of it?
And then you gain access to the points?

If these points have been conterminated with oil, this will lead to the injectors not fireing at all, or they will fire, but with wrong characteristics?

Because, as I have indicated, using a screwdriver as stethoscope, they all sound alike.

As I have understood, it is possible to test the MAP sensor by applying a certain vacuum level and then measure a resistance, right?

Hugin · #21 06-08-2009, 04:19 AM

BTW, will a leak from the vacuum-driven locking system influence the performance of the car?

My locking-system will keep it's vacuum for several days if I am not mistaken.

Also, found a description on removing the distributor....will do.

Hugin · #22 10-26-2009, 09:19 AM

Hi

Have replaced map-sensor, car is running better now.
Does the map-sensor need adjustment?

The car idles good, but backfires very often, both on idle and higher revs.

Any good suggestions?

Have tried to adjust ignition, no good effect.

Takernz_30 · #23 10-26-2009, 10:27 AM

Quote:

Originally Posted by bwostosh

Why do they put shipyards in the tropics?
I have done Avondale, La. (lofting group)
I could not believe those guys with full leathers doing arc all day

I think it's the location b/c the seaside is very deep, it's ideal for shipyards and away from typhoons.

The welders there were pretty used to the climate, I was there for 2 months as my part of my training, I used to bring along my canteen when going to work as a welder.

Graham · #24 10-26-2009, 12:15 PM

Quote:

Originally Posted by Hugin

Hi

Have replaced map-sensor, car is running better now.

Does the map-sensor need adjustment?

You can't really adjust it without an exhaust gas analyzer (at least a CO analyzer) or as I have done, a wide band AFR gauge.

Quote:

The car idles good, but backfires very often, both on idle and higher revs.

Any good suggestions?

Have tried to adjust ignition, no good effect.

The engine should never backfire!

If you still have points, you should set them at 0.014" for a start and later check dwell is correct (I forget the number - maybe 30-34deg?). But better still, replace them with a Pertronix 1885 (leave MB switchgear and coil in place for a start - do a search - there is info here on how to do this.)

Trigger points sometimes just plain wear out and then they fire the injectors incorrectly. (I am working on a way to repair old sets, but not quite there yet

). Other times, they just get oil on them and need cleaning - Just spray them with some electronics cleaner and run some paper between them. When you put them back, make sure they go back in properly so they seat against the distributor body.

Once you have them back in place, set the timing. It should be set to about 5-8 deg BTDC with vacuum disconnected. (Distributor turned almost all the way CCW)

A couple of other checks - have a look down inside the throttle body into teh intake manifold - Is it clean or sooted up? Also pull the plugs and see what they look like. Maybe post pictures.

Finally, read the section of the manual (7.4, I think) that covers checking all components of the FI system. You can check it all with a simple ohm/voltmeter.

Good Luck

Tomguy · #25 10-26-2009, 02:50 PM

When you say backfire is it a fire out the exhaust (raw/unburnt fuel igniting in the exhaust from the heat) or actual backfire out the intake?

If you're backfiring out the intake, you may have a serious cam timing issue. Check chain stretch, make sure that's all good.

If it's misfiring and burning unburnt fuel in the exhaust, your new MAP is probably set too rich - as in set for a car that may not have had any vacuum leaks where you do, or just for differences in that engine's setup.

Please tell me you cleaned the EFI trigger points! On my car this is something I had to do monthly when I drove it daily. I think my rings were gummed up from sitting so long, which contributed to higher vapor pressures in the crankcase, but it is still a regular job that should be done on an ongoing basis.

Hugin · #26 10-26-2009, 03:51 PM

Hi Tom

It's backfireing through the exhaust (with a "sucking" sound from the intake).

The engine revs fine in neutral, but under load it will not rev up very much.

I have checked spark on all cylinders (all parts new) + checked fuel pressure ok.

I can hear all injectors click using my ear to a screwdriver.
Will make it my first job to clean points, promise ;-)

Have no CO-meter, might need to take it to a shop to get this done.
Have tried a "by the ear" MAP adjustment....no big success.

Very greatful for your feedback Tom.

Will post result of cleaning, hopefully done during weekend.

BTW, have bought a complete heater-control unit, is it a big job to exchange it?
Have a complete set of mint woodwork, I was thinking of replacing the woodwork at the same time.

Tom

bwostosh · #27 10-27-2009, 10:53 PM

Check the spark plug color for rich or lean condition
or plug wires for cross-fire. Then adjust map sensor to suit. This is after EVERYTHING else is correct.

Heater unit is a big job, I'd rather pull a motor

Hugin · #28 10-28-2009, 07:03 AM

I have 4 broken levers (handles) so there is no way around it.....

And they are HARD to move.

Is there a "manual" with pics available?