I’m stumped! Need some guidance SL500 R129 1990 misfire

[D12typ]

Evening all, been reading various threads and obtaining as much info as possible but I’ve lost the will to live whilst trying to diagnose what I’d expect to be something quite straight forward

Spec. 1990 R129 SL500 V8

Issue,
-very slight misfire at idle (maybe just rough running at idle)
-under acceleration from standstill the car bogs down and struggles to accelerate then surges into life
-under full load feels as it should and has what seems to be full power with no issues

Replaced and inspected Plugs - replaced Leads - replaced Rotor Arms - replaced Distributor caps - Bosch, cleaned and pass under visual inspection Coils - both (x2) read correct vehicle voltage on both poles 1&15 EZP - continuity between plug A terminals 1&6 at both coils
The fault feels like an air mass meter / flow meter related fault but this vehicle doesn’t have one so I’m stumped, it’s a friends car I’m looking at and want to help get it fixed
Any help would be appreciated
Thanks

Just to add, visual inspection of wiring loom, not sure If the fault applies to this year but apart from the shroud being a little brittle the wiring seems ok from what I’ve seen
Was contemplating a Crankshaft position sensor due to a little oil ingress on wires but car starts and run which would suggest this isn’t the fault
On idle with airbox covers removed there is a hissing noise from throttle body which again I assume is the norm as it’s at idle
Really struggling to work this one out, symptoms of an EZP? (Ignition control) or coils?

 

[peterws1957]

Could this be a fault with the wiring inside the throttle body? Have a look at vsandvs videos on YouTube who seems to be an expert on these. Many vids on his channel showing how bad the wiring can get.

 

[D12typ]

Thanks for the response, I’ve just watched a few and they seem to all have an associated ASR light appear on the dash which this one doesn’t have, can’t believe how many gremlins these cars have

 

[LostKiwi]

Your car shouldn't be affected by wiring issues as I believe these started some 2 -3 years after yours was made.
As an example of how bad the wiring can get this was mine...

Believe it or not no misfires or other symptoms.

Have you tried ignition coils? They're about £50 from memory from ECP or GSF.

 

[D12typ]

I have bench tested them and they are ok at idle but maybe that needs to be the next purchase, it may be that under load they are failing.

Thy wiring above looks dreadful!

 

[LostKiwi]

Have you had it on Star (yes Star will read faults on it)? You can also sometimes get good info from a blink code reader (which is cheap and easy to make).
From what I recall the EZS has its own module for storing codes - pin 17 on the 38 pin connector.

 

[D12typ]

No I seem to have every adapter for my diagnostic equipment except the one which this car requires!

May have to find someone local that has the STAR as I imagine it’s packed full of faults. Didn’t think a car of this age would give much back via diagnostic feedback though

 

[LostKiwi]

Not a huge amount but you only need one pointer in the right direction!

Where are you based?

 

[D12typ]

I’m in west London / Surrey, place called Sunbury

 

[Flyinspanner]

Go to Wayne Gates in Harrow. Top Indie.

 

[D12typ]

I may have to,

Update, and question, I unplugged the EHA and it made no difference whatsoever
I’d expect a change in engine tone or performance but it still runs rough and is very hesitant with misfiring and popping from exhaust

Wide open throttle seems to find its life again and I can’t fault it

So does anyone know if I should have a change when the EHA is unplugged?

If the case is yes then I may have a lead

 

[LostKiwi]

The EHA is only a fine adjustment on mixtures. I wouldn't expect too much change by disconnecting it.

 

[bembo449]

so is this a carb'd V8 unit ??

 

[LostKiwi]

so is this a carb'd V8 unit ??

No, electro mechanical injection.

Bosch K-Jetronic series.

 

[D12typ]

Damn, thought I might have been onto something with the EHA

I’m heading towards replacing the ignition coils to see if they are the cause, trying to avoid throwing money at it and would prefer to diagnose the fault for sure

 

[bembo449]

think id take the advice and book a star sesh with one of the recommended above pal , its the way forward rather than guessing , although it reads like the timing is out to me

 

[LostKiwi]

How many miles has it done? Does it have lambda sensors?

 

[Uncle Benz]

Sounds more like fuelling to me from your description that it pulls well on wot. Injector spray patterns can often be poor on these, and might be somewhere to start. It doesn't feel like spark to me from what you have said

 

[LostKiwi]

Sounds more like fuelling to me from your description that it pulls well on wot. Injector spray patterns can often be poor on these, and might be somewhere to start. It doesn't feel like spark to me from what you have said

I'm thinking that if it has lambda feedback it may be fine when running open loop (WoT and cold) but poorly when closed loop (normal temperature and partial throttle). Should be able to check lambda values via Star I'd think.

 

[mattkh]

What does EHA stand for?
My eyes lit up at the mention of Bosch K-jet.