engine problem

Dear All,

First of all I wish you a Merry X-mas and all the Best in the New Year.
Hope the vaccinations will start and we will get back to 'normal' as soon as possible.

I also would like to thank the moderators and to all contributors for the support you give to solve our problems.

I have the MB 116.982 engine 3500 cm3 V8 with D-Jet. (w107)
When I start the engine the engine starts around 1200-1300 rpm when it is cold.
(Last year I had the auxiliary valve repaired as the cold start rpm would reach 1800-2000 rpm.)

If I try to increase the idle speed by pressing the gas pedal in order to warm up the engine quicker I hear a thumping (like a small explosion) sound at certain rpms ie 1800 or 2200.
It feels like the engine misfires. It also creates a strong vibration.
Once the engine warms up this phenomenon disappear
I went up to 6000 rpm without any problem.

After a long ride (around half an hour on the highway at speed around 90-100km/h) when I slow down and the rpm goes down to 1500 rpm the engine speed starts to fluctuate around 100 rpm up and down but it is not as strong as the sawing effect when the engine is cold.

Previously I was told to check the vacuum in the engine.
I got hold of the vaporizer and the vaporizing liquid but did not have time to do it due to the pandemic restrictions especially in the week-ends.
So this is the next thing on my mind If this is the only way to find out what is wrong with the engine.

regards to all of you and happy holidays
juan

Hi Juan,

a warm engine rpm of 1500/min is far too high. It should be 700-750/min. Cold it can reach 1000-1200/min, not more. A warm engine should also have a vacuum of at least -500 mBar. It would also be interesting to know your CO and HC values at idle.

What you write about misfire sounds like a wrong ignition timing or mixture is badly wrong.

BTW Kutluhan and you principally have the same engine. So you can swap components for testing.

Hi again,

my warm engine rpm is set around 700 rpm.
actually the mechanic set it lower last time and he said this would eliminate the problems until engine warms up.
when I shift to D and stop at the traffic lights the engine rev. goes down very low but does not stop.
I asked whether he could raise it just a little bit he said he did not want to do it because of cold engine problems I have mentioned.
I had to go for TUV test for the homologation this week:
the exhaust values are (volume %) and I do not have the catalytic converter.
CO 3,62
CO2 7,84
O2. 0,34
HC 167 ppm

Hi Juan,

if your engine rpm cold is too high and warm okay, then your AAV is suspicuous.

Your misfire can well come from your ignition distributor. I had a case once where it would not adjust timing properly when cold. Warm it was okay. That created a bad misfire and that 350 SLC would cold not run faster than 50 km/h. Warm it went normal. I only noticed when I left that car in my cold workshop overnight and hooked it up to ignition tester cold next morning.

Hi Volker,

You did not make any comments on the exhaust values from which I assume the values are within an acceptable range.
Before I got the AAV repaired the cold engine rpm went up to almost 2000 rpm.
After the repair it is around 1200 rpm. I do not know the cold rpm values but it definitely went down.
I did not try to go faster than 50 km/h with a cold engine and in the milder Istanbul climate engine warms up quite fast.
I only tried revving up the engine while the car was parked and I heard the misfire sounds though not continously.

Anyhow thank you for your comments,

Reagrds,
Juan

Hi Juan,

1200 rpm on a cold engine in Istabul seems a bit high, especially if your ignition timing seems so wrong. But that is a minor issue now.

I propose that you verify ignition timing on cold engine in idle, 1500/min and 3000/min. Also watch if you have varying/jumping ignition timing.