Hi there --

The engine -- a Euro M110.984 -- has just had a rebuilt head.

It's got a rebuilt fuel pressure regulator, fuel distributor and warm up regulator.

The timing is correct. Fuel pump seems okay. There's a new cold start valve and the auxiliary air valve seems okay, too.

It's a 78 so no oxygen sensor.

Regardless of how the CO mixture is adjusted -- toward rich or lean + I wish I hadnt touched it -- Im getting these issues:

Starts well cold.

Some backfires when the engine is revved.

Begins to die when warm

Then hard to restart.

Does anyone have any advice? Or maybe know of anyone who can do a metering/ fuel pressure diagnosis in the Brooklyn area?

A few more words of identification would be helpful;
1977-79? No O2 sensor.
1980-81? W/ O2 sensor.

Re: Fuel pressure regulator (FPR). The FPR is a component of the fuel distributor (FD). Was the whole FD rebuilt? Could it be that the warmup regulator (WUR) was rebuilt, rather than the FPR/FD?

Re: Distributor. The ignition distributor or the FD?

Begin by checking the valve clearances; make sure that they are not tight.

Hi there -- thanks for your message.

The FD and FPR was rebuilt as a whole.

The WUR was also rebuilt.

Both by Hans at H&R out on Long Island.

It's a '78 so there is no 02 sensor.

The valve clearances are good.

Re: Backfiring. Is the popping in the intake, or in the exhaust (aka afterfiring)?

Re: Ignition timing. Is the distributor providing appropriate centrifugal advance? As a check on timing, remove both vacuum lines from dist. and plug them. At idle ~ 10deg. BTDC. At 1500rpm ~ 20deg. BTDC. Above 3000rpm ~ 30deg. BTDC. Advance vacuum should add ~ 12 deg. for a total of ~ 42 deg. above 3500rpm.

Re: Cyl. head rebuild. Has the cam timing been checked and verified?

These questions are asked because errors in both cam and ignition timing can lead to chasing after injection "problems" that aren't. That said, if cam and ignition are eliminated, the next most likely source of the symptoms you describe is in the WUR. If, as the WUR increases in temperature, it closes too far, the control pressure will become too high and the metering plunger in the FD will be held too far in the lean direction.

Thank you again.

The backfiring is just popping in the intake.

Yes -- the cam timing is correct.

Not sure about the advance. The distributor looks in pretty good shape

I'd say the WUR is the likely culprit as you suggest. If remove the power to it when the engine is running, should I notice any difference?

I tried this, and it seemed to make no difference....

It sounds like a classic fuel distributor issue to me. Have you checked the pressures in and out of it? There's a whole diagnostic process that you can perform with a fuel pressure gauge.

see here for more info: Bosch D Jetronic and K Jetronic Troubleshooting - Benzworld.org - Mercedes-Benz Discussion Forum

__________________
With best regards

Al

Re: Advance. Do try to confirm that the centrifugal advance is correct; it can have a significant effect on throttle response.

Re: WUR. Unplug the WUR heater connection from cold. Eventually block heat will raise the WUR temperature, however, the delay will enable you to tell if the WUR is the culprit.

At the risk of stating the obvious, do check that the spark plug wires are in the correct firing order.

Thank you for all of this. It sounds like I need a pressure gauge.

Is there a simple way to test the WUR?

At this point some pressure and flow information will be necessary.
One of the pressure tests will be for control pressure, which will reveal the response of the WUR at a number of temperatures.

The quick but somewhat crude test of the WUR is in my last post.

Hi there --

So I hooked up a pressure gauge to the Fuel Distribitor and the Warm Up Regulator.

It read 2 bars at cold, and ran fine.

By the time it was warmed up, it was reading 3.6 and beginning to die. And then it did 4 bar.

Meanwhile with the tap switched to off, the pressure read a fairly consistent 5.

Is it possible the WUR is not receiving any current? It didnt seem to make any difference to the pressure reading when I unplugged it with the engine running....

What do you make of that? Is that, in fact, rest pressure?

Best Ed

Control Pressure, measured in the line between FD and WUR, at 20 deg.C,
should be 1.0-1.5 Bar. At 40C. (WUR temp) less than 2.5 Bar.
If you are observing up to 4 Bar control pressure, that is almost certainly the source of inlet popping and shut-off, due to extreme lean mixture. If the WUR were not receiving power, the temp. and the control pressure would be lower (until engine block temperature increased).
What are the last three digits of the Bosch number on the WUR?

System Pressure, measured at the supply inlet of the FD, of 5-5.6 Bar is normal.

Hi Frank

I tried again yesterday and much the same thing happened.

When its warm and I shut off the WUR using the tap on the pressure gauge, the engine runs better.

If I remove the airpipe from the manifold to the WUR, the engine speed increases.

Leads me to think the engine is too rich, not too lean.

Still hard starting when hot, tho.

Is this at all possible??

[QUOTE]"When its warm and I shut off the WUR using the tap on the pressure gauge, the engine runs better."

By closing off the control pressure from the WUR, you are reducing the control pressure at the metering piston in the FD. Reduced control pressure results in a richer mixture.

[QUOTE]"If I remove the airpipe from the manifold to the WUR, the engine speed increases."

When you remove the airpipe (vacuum line?) you signal the WUR that the manifold pressure has increased to atmospheric pressure, which is a full load condition, and the response of the WUR is to reduce control pressure to provide a richer mixture.

Please note that in both of these scenarios, improved running of the engine is the result of providing a richer mixture.

Does it have spark when warm?

__________________
600SEL '91
300E 4Matic '88
240D '83
280SE '77
350SE '73
The most complex systems can fail in the simplest way.
Contra verbosus noli contendere verbis, sermo datur cunctis, animi sapientia paucis.

i don't believe in the lord! He's never bought me a Mercedes Benz.

Yes it has a spark when warm.

Hi Frank, I'm beginning to get the picture.

High pressure signal from WUR leans out the FD to the point that the engine stalls and will hardly hardly restart.

(And this is despite the mixture, I would say, being pretty rich at cold idle. Certainly smells like it's dumping gas in there.)

What do you suggest I do next? As I mentioned earlier the WUR is a recent rebuild from H&R in Long Island.

I guess it should go back to him, or can I look to another solution?

Best Ed