W126 Huge Misfire help please!!

[mercw126man]

I have a 1991 W126 420SE with the 4196cc V8. It has the misfire from hell when over 4500RPM, with a kangarooing effect which feels to be across all cylinders.

I've just had it into a local Merc specialist (Merc-Care of Leeds) who said they thought it might be either the Fuel Pump Relay (FPR) or Transistor Pack. Only charged me for an hour's labour, but I really wanted it fixing and I could have guessed at the likely cause myself.

Have replaced the plugs (all correct colour) and the engine is otherwise very healthy.

Where is the FPR situated, and does anyone have any experience of anything similar?

Thanks in anticipation.

 

[television]

OK relays can be a source of problems The relays are in the fuse box under the fuses, in my veiw I would have thought the pump or fuel filter would be more the problem as the relay would also effect the slow speeds and I would have thought that the transistor pack would have shown up on speeds well under 4500. Is it missing or backfiring.

Malcolm

 

[mercw126man]

Thanks very much for your prompt reply.

No, apart from the huge 'on/off' misfire at 4500-4750 RPM plus, the engine is perfect. It feels like a fuel starvation problem (but is very dramatic and nearly catapults you through the windscreen), or an ignition system fault, hard to say. I believe the same relay controls the kickdown and the fuel pump, could it be that relay? I'd have thought that it would show some symptoms at other revs, not just when demand is highest? Similarly, I would have expected some misfire at other revs as well (like when its pulling uphill on wide-open throttle but low revs)?

Cheers.

 

[jberks]

but I really wanted it fixing and I could have guessed at the likely cause myself.

Daft question but why didn't you ask them to fix it then?
On the 2 occasions I had a misfire on the E (that the dealer couldn't suss), I gave it to them on the agreement that they wouldn't return it until it was fixed. Both times, came back sweet as a nut and I only paid for a little extra labour to figure out exactly what was wrong, and just the parts I needed.

 

[mercw126man]

Yes, a fine question. Sorry for the 'war & peace' that follows, but you did ask!

I had to take a half day off work, then walk around Pudsey in the rain for 4 hours while they worked on it. They had spent 'around an hour' looking at it, and didn't ring me when they had finished as I asked. I turned up at closing, and hope they had it ready. It was, but not fixed. They had told me on the 'phone beforehand (I booked it in well in advance) that they had loads of cars they could borrow parts from, so shouldn't have any probs/diagnosing fixing it.

They gave me a bill for £41 (very reasonable) but said they could find nothing wrong with the diagnostic machine(s). The bill was marked 'unable to find fault within legal speeds'. I thought that was the purpose of a gearbox, but there you go. Uphill, or in bottom gear, would have produced the fault. They hinted that I must be some sort of 'boy racer' to even notice the fault in the first place. In my defence, I was told about the fault when I got it (from my Dad), who is retired so probably doesn't fit the 'boy racer' profile. I'm a middle aged teacher, by the way, so I might not either.

Anyway, I'd rather have paid more money, had them fix it (preferably within the 4 hours) and then driven away a happy bunny. They suggested I re-book it back in, for two days (hinted wouldn't be two days labour, necessarily) and they would try and identify/eliminate the fault.

Being fairly adept with the old spanners myself (engineer in a former life), and not really relishing the thought now of a massive bill, I've decided in the meantime to do some research of my own, to try and fix it myself, hence the questions. Don't mind paying for expertise, as long as it gets a result.

Anyway, sorry for the 'rant' but I'm no further forward. Am considering buying another complete, running car as a donor vehicle, as it might be a good way of fixing this fault and stocking some other parts, at the cost of one new part (can get one on good old EBay for a few hundred).

Thanks for the info; I suppose if I get nowhere I'll have to take some holiday and book it back in. I do feel many garages want the easy, 'fitting' work such as servicing or brakes, but shy away from the more difficult jobs and felt I was someway down the pecking order with my problem.

 

[television]

Lets take this one stage further, and ignore what MB told you. The relays do give problems, but not your type

There are two test conditions, Driving and static, and two possibles, electric or fuel. If it was my car I would go for fuel.
First electrics, we have the primary and secondary circuits in the ignition. the primary side does not know or care if the car is static or driven, the secondary does, Direct ignition (coil bank) can break down under load, but not all of them, so if it was that only one coil would break down causing the car to hesitate and not cut off at xxx revs. It is not possible to measure or test them, sinse 1 short turn would make the coil useless, and you cant measure 1 short turn in 20.000. If you have a normal coil it would not give you the problems you have,it would just get hot, and the engine would stop.

On to fuel, Last month a friends V8 Rover was fine on the level but would not go up hill, That was a blocked filter. A guy who does work for me had a Renault 19 that was fine up to 30, over that and the engine died, he bought a £4 plastic in line filter and the car is fine. With fuel injection you have two punps, we can ignore the high presure pump.
You have a in line filter connected to the pump( service item) and that's just in front of the tank, drivers side, I would change the filter and when it is off you can test the pump. put a bowl to catch the fuel have someone turn on the ignition and the pump should click and pump around 20 times in 10 sec.
The Symptons would differ between fuel injection and carb, if there was not enough fuel with a carb the engine would splutter but not cut, with fuel injection the engine would cut off as there would be no fuel to open the injectors.

Malcolm

 

[mercw126man]

Thanks Malcolm,

I'll change the filter, check the pump output and report back.

Thanks once again.

 

[television]

Change the Pump its a Bosch 910 along with the filter. Make sure it is in the Yellow box. You can compare price between a bosch factor and MB.

Make sure it is not made in czech

Malcolm

 

[jberks]

I had to take a half day off work, then walk around Pudsey in the rain for 4 hours while they worked on it. .

Fair enough, Pudsey isn't the most exciting place to spend 4 hours! If you do go back, take one of their loan cars. I took one and kept it until mine was ready. That way they could take as long as they needed (I told them I didn't want it back until it was right, however long it took). I initially had a couple of visits like yours, which is why I took that tack. They are pretty straight in my experience and only charge for what they actually do, just extremely busy so yes, from experience in 4 hours they won't get very far as I doubt they spent more than 1 hour on it (hence your bill). The comment of swapping parts is true, they fitted a throttle actuator to mine, it didn't work and the swpped it out again - try getting a dealer to do that!
My misfire was very intermittent so I gave the boss (Ken?) my car, and asked him to use it as his own until he found the fault. Took 2 days before he found it apparently , but he did find it and subsequently fixed it. I got a call 4 days later to say she was ready and she came back sweet as a nut. Certainly persevere on your own, but don't get dragged down the slippery slope that many do, of spending money on parts that may or may not do the trick, that's where their ability to swap bits in and out really pays dividends.
Good luck,
JB

 

[television]

Sinse when would a throtle actuator stop a missfire.

I do not normally tell people what to buy,but I did think about this fault and it only left the pump. This is also a common problem with early XJ6 cars. I found 10 other cases this morning 126 Cutting out above 4k on all it was the pump.

Malcolm

 

[mercw126man]

Thanks Malcolm, I'll spend some time sourcing one. Any suggestions (Euro Car Parts?)

 

[television]

Big problem with Euro parts where do they get their parts. some people in the USA had problems with cheap imports and only solved their problems with a bosch 910 in Yellow box. Look in your phone book, motor factors and find one the handles Bosch.

Malcolm

 

[mercw126man]

Thanks for the comments and tips JB; I wasn't trying to run the dealer down as the other side of the coin is that they often get the ****ty end of the stick with the hard to diagnose problems. It's just that when you go back full of expectations, get a bill but no progress you feel somewhat disappointed.

I'll change out the pump and filter and see where we go. Thanks again for your input.

 

[mercw126man]

Malcolm,

Does this vehicle have twin fuel pumps? There are two that appear to be inline with one another, then dispensing via the filter. I bought 'the' pump, do I now need to buy another/are they different from each other?

Thanks,

 

[mercw126man]

Twin or Single?

Malcolm,

I've bought a pump for the W126, but find that the car has two pumps fitted. Are they both the same, as they appear to be in series with one another, and then dispense thro' the filter? Do I need to buy another to be sure (having two increases the chance of one not being operational).

Thanks.

 

[television]

Just been looking and as you say you do have two, this means that one is US. The two pumps are the same, only one will be faulty. the way to find the faulty one is to take off one cable from one pump, make sure it cant short or touch anything, and try the car, if it runs as before,put the wire back and do the same with the other pump, with luck one of the pumps has packed up. Come back before you change one.

Malcolm

 

[davidsl500]

having reread mercw126's original post I am somewhat confused.

If I understand it correctly then the car behaves impeccably up to 4500rpm but over that it behaves like an engine possessed..

Now I presume that this is 4500rpm in any gear and in any throttle condition ie., part or wide open throttle. The common denominator is therefore the engine revs since the fuel demand will be different dependant on gear and will vary also with throttle position. So if the problem was fuel related I would have expected the misfire to occur at different engine speeds dependant on gear selected and throttle position?

To check out the theory, assuming it only misbehaves at 4500rpm then how about running it down a good hill on overun and then applying the smallest amount of throttle to get it over 4500rpm and check the result - if it misbehaves when there is little fuel demand then I would suspect an ignition issue. Its almost as though an agressive engine speed limiter is kicking in.

I do hope that the pump you have bought sorts it, its just that I have this suspicion that the problem lies elsewhere. Good Luck!

 

[mercw126man]

David/Malcolm

Thanks for the posts. I agree with your thought processes, and indeed the car does not miss when using full throttle when going uphill, but with revs lower than 4500, which I would have expected as fuel demand will still be high. However the bits I don't understand are how the various fuel system components are controlled (or not) at high engine speeds, so I am seeking your advice on this. The dealer seemed to suspect the fuel pump relay, and they had seen a batch that refused to work above 3000 RPM, but again I don't understand how this is in any way linked to engine speed. I would have thought the relay was either on or off?

I'll see if I can diagnose a non-operational pump by substitution. I assume the faulty one will just act as a 'pass-through' for the other?

 

[television]

After my thought process I had a look on Benzworld site, there are a lot of postings, over 4k engine cuts, answer, Fuel pump.There are no postings for Ignition faults

On the MB WIS system it say's that the surplus fuel valve sticks open, sending the fuel back to the tank and starving the engine = change pump.

You have two pumps, it different story to cars with one, and the two cars I spoke of. At 4.5k revs your car needs a lot of fuel, On a carbureter car when staved of fuel the odd bit comes through and it will hesitate and hold back. With injection, if the high presure pump cant get enough it cuts,and no fuel comes through. The fact that you can rev the enginge when not in drive shows that the ignition is OK.

I suspect that you are only running on 1 pump.

If you do change it, run the tank low, and jack up fuel pump side and put a bowl or bucket under the unions to catch the fuel, it can be sived and re used

Malcolm

 

[television]

AS it stands its simple to test

Take a wire off one pump and try the car, if car drives OK, put that wire back and take a wire off the other pump and try the car.

if you have a faulty pump you will find it by doing the above, with luck the car will not go on one of those pumps.

It needs two pumps, as one cant supply enough at high speeds, and its a big engine
The only limiter I can find is a max speed linked to speedo

Malcolm