850 20V 170PS rough engine running and 3rd cylinder isn´t building up exhaust temperature

[KptnChaos]

Hi,
I have a problem with the engine of my wife´s 850 sedan. After engine rebuild it has a rough running engine and when checked the 3rd cylinder is not building up exhaust temperature as the rest.
3rd cylinder 80°C (176°F), the rest around 200°C (392°F) in idle after appr. 30 seconds.
If I unplug the injector or the spark plug of the 3rd there is no change in engine idle. If I unplug one of the things on another cylinder the engine nearly dies.
The compression is 10,5 to 11 bar ( 145 to 160psi) on all 5 cylinders.
And there is a smell of unburned fuel in the exhausgases

Here is what I tried so far without any result:
- new spark plugs
- new ignition cables
- swapped complete ignition system from my other 850 in this one
- changed injector 3 with 4
- changed spark plug 3 with 4
- checked the resistance between the plugs of the injectors
- checked with an endoscope whether the valves are opening. They do
- pulled out the fuel rail and checked if all injector spray fuel
- changed the engine control modules (I had a spare set laying around)

When you rev up the engine the 3rd cylinder builds up temperature, but significant less than the others, 3rd 150°C to 350°C the rest

The unique feature of the 20V with the jetronic is, that all injectors are actuated at the same time.

I ran out of ideas of where to look and hope, that someone has an idea what I overlooked.

Have a nice weekend, Martin

[misha]

Jetronic will have an additional ignition module in ecu box.
Read the codes from it(A6 port) and engine control module(A2 port)...from both.
You will have diagnostic ports under the hood.

https://www.matthewsvolvosite.com/volvo ... 5-to-1998/

[KptnChaos]

Hi misha,

sorry did forget to wirte it in my post.
Both are free of faults. Both show the 1-1-1.
I wish they sowed an error. Then I would have a point to start at.

The engine starts and you can drive it. It accelerates. Although both idle and at higher rpm ist doesnt run good.
And in idle it smells like unburnd gas at the exhaust.
The 3rd cylinder is working. But not at 100%. And with this I wont pass the emission test to get the car on the road again

When I unplug the injector the exhaust temperature is at 35°C. When pluged in at 80°C. So a litte combustion takes place. But I havent got the slightest Idea of where to start looking after all the thing I´ve already tried.

[misha]

Could be bad injector or bad fuel pressure...since compression is good on all cylinders.
Check fuel pressure regulator...it could leak raw fuel in vacuum line.

Try with disconnecting Maf sensor...if it runs better...you found the culprit.

[KptnChaos]

I had interchanged the injectors but the fault stayed at the 3rd cylinder.
Pressure regulator and Maf sensor would effect all cylinders as far as my understanding goes.
I will open the camshaft cover and check the hydrovalves. Nothing I wanted to, but I´m running out of other options of where to look.
Thanks for the quick answers