300TE misfire

[Holyman]

I have a 1988 300TE with 140k on the clock and it has a misfire. Sometimes it will run a couple of days without a problem, other times it misfires constantly.

Occasionally this is accompanied by a fluctuating idle and today, for instance, it kept cutting out in traffic.

I have changed the plugs, leads, rotor arm and distributor cap.

What I have found is that if I unplug the Jetronic ECU and re-connect it, for a couple of days I get no misfire but the car feels sluggish.

It seems to run better at high speed - constant 100mph cruising is smooth and feels good.

Any pointers would be most welcome.

Also, what fuel consumption can I reasonably expect?

Thanks in advance for ANY ideas.

Martin

 

[Rog_E]

Hi.

My '88 300TE 4-matic has similiar behaviour, runs Ok for a day or two then will 'hesitate' at speed (60 - 70 usually ) and from time to time will cut out all together, usually when negotiating a tight 'must have power steering' turn.

The idle also goes nuts at times - will spontaineously rev to 1500 rpm at idle (even tries it in drive at a junction).

I have changed the plugs, air filter, distributor cap, fuel pump relay and ht leads - no change.

If I turn the ignition off and then bak on it runs fine again - does yours do the same?

The behaviour does indicate a relay fault - it resets after ignition off, but this may be me hoping it isn't the ignition module.

I will spend some time next week (I'm taking a week off) trying to resolve this but any hints will be most welcome!

 

[joe bloggs]

First thing that springs to mind is fuel pump, the pressure on these injection systems is critical, usually they have 2 pumps, are they noisy?
you would have to screw a guage in to check pressures, friendly dealer may do it for you, only takes 5 minutes.
Try pulling the plug leads off one at a time while the engine is missfiring to narrow the cylinder down (loosen them first and just lay them in place) not sure why unplugging the jetronic unit should affect it, does it run ok with the unit disconnected ? is the mixture (Co.) ok?

Rog, yours sounds more like the overvoltage protection relay occasionally not supplying current (dirty contacts, which when you switch off/on may be making a better connection) - yours will have several contacts/outputs for the jetronic / abs / 4 matic systems, try gently tapping it with the engine running & see if the engine reacts (it is fitted next to battery - the one with the red fuses on the top) new one is about £50

Good luck guys

 

[Holyman]

Yes mine sounds like the same fault but I'm not sure about it clearing after switching off the ignition and restarting.

I took the plunge and put the car into a 'specialist' this morning - not paying main dealer prices - and he just phoned me with some unusual news.

Appears that the rotor arm is no longer positively connected to the camshaft - the retaining bolt has sheared off, so that the rotor arm is free to slide on the camshaft.

This will be fixed today and then we'll see if it continues to play up.

I have read somewhere that the OVP relay can cause ignition faults - loss of spark etc. I guess that is a simple thing to replace for starters. I'll let you know how we get on!!

 

[Rog_E]

Thanks Joe!

I thought it might be the OVP, didn't occur to me to give it a tap and see what happens!

When I change the fuel pump relay I did notice a second relay beside it, which I also removed/reseated.

However I don't recall it having red fuses on top!

This may explain why the 4Matic engaged warning light comes on briefly when I get near 80 (originally thought it was a 'speed' warning :lol: then I read the owners manual ) I'll give it a go.

Best to buy an original?

Holyman - loose rotor arm!!! Amazing it didn't take out the distributor cap! Sounds a bit suspicious to me.......
Good luck.

 

[Rog_E]

Found the OVP relay.

Hoped I could remove the outer casing to check the condition of the contacts inside, but chickened out when my local MB dealer told me it was not in stock.

Ordered a new one, £42+VAT, it'll be here in the morning.

Replaced the current OVP and the car started much better!
No hint of a misfire when revved and the tickover seems smoother.

Could be being subjective though!

Fuel Consumption:

23-25mpg mainly rural and relatively heavy right foot!

 

[Bruce_Jeffares]

Holyman;

I reckon Joe is on the mark with fuel pumps. I had both replaced within about three months, during which the symptoms were just as you describe.

Did wonder if I was being taken for a ride; two fuel pumps ! what sort of a car is this !

Expensive pill, but fabulous drive afterwards.

 

[Rog_E]

Changed the OPV as suggested by Joe.

So far so good, although a bit early to tell for sure, no more hesitation or idle fluctuation.

When revved to 6k+ rpm the engine no longer misfires.

Bruce - as a fellow 4-matic owner do you experience the 4-matic engaged light at about 80?

Mine comes on, briefly and intermittently, at almost dead on 80.
I'm guessing that this is not designed behaviour but may be down to a Hall sender /stator fault on one of the front wheels.

 

[joe bloggs]

All tyres the correct size? daft but common, also better to have same make on same axle, any slight circumference (wheel speed) differences will show up at higher speeds. ABS not so fussy
Also whip the wheel sensors out next time you are under car and give them a clean (may be tight, leave them if they are solid as you will break them and they are not cheap. The serrated sections on the driveshafts can corrode away also, check with a torch while turning shaft.
Is it the w/lamp or the triangle coming on? Does the ABS give a kick when braking to a halt?

 

[Holyman]

Well I finally got the car back today - it took the local Merc dealer 3 days to come up with the right 70p bolt for the rotor arm carrier - ho hum!

The report from the guy that did the work was that the misfire is a whole lot better now that the rotor arm is attached to the camshaft but there is a rough idle due to a dodgy lead on no.5 plug. When you listen to the ebgine idling there is a sharp 'crack', which he reckons is the lead arcing, followed by a quick rise and fall in revs. The engine is now fluffy pulling away but is very smooth and responsive under load - so a minor result there.

He also reckoned that the CO was set to 4.5% in order to make the idle better and the misfire less obvious. This has been reset now and may improve fuel consumption a bit.

So a new plug lead next - I'll see if I can improve the existing one first.

 

[Bruce_Jeffares]

300TE misfire; 4-Matics

Rog_E

No problem with my 4-Matic warning light.
It will do the business crawling through slippery mud etc.
It appeared once when I was sneaking up to three figures and happened to crest a small rise in the road; sort of take-off warning.

Sorry I can't reassure you, but there is no obvious link with speed.

Pity there are not more 4-matics around; everyone could have one !

Bruce

 

[Rog_E]

Joe:

I have noticed that the ABS can kick when pulling to a stop, usually just before the car comes to a rest.

I haven't checked the tyre sizes, four new tyres were fitted just before I bought it, but I will check!

My money is on the sender/stator still.

Bruce:
4-matic was wonderful in the snow (I also have a Discovery but prefer the Merc), and I agree there should be more of us!
Here in Cambridge there are some narrow badly surfaced roads - the 4-matic light can come on when I get enthusiastic!

Back to topic:
Since changing the OVP the problem has all but disappeared.
I have over the course of a week only experienced two mild interruptions to power and one case of the idle going nuts (idle control valve?).