thepurpleblob

Well.... I think it's a missfire.

1986 Euro 16V with twin distributors - I'm working through a long list of niggles.

The car has a bit of a missfire. It always has. The missfire is not there when it's cold, only when warmed up. It's a very bassy "wumph, wumph" (from the echaust) that makes the car rock. It's constant but feels quite random. I tried to record a video but you can't hear it - the sound is too bassy. Revving it up doesn't help - I can still feel it.

I wouldn;t say the car is down on power. It pulls well. It has new leads, caps, rotors, plugs. All vacuum lines, o-rings and connectors are new. The injectors have been flow tested and cleaned. Fuel pipes are all new.

I am aware these cars are quite "boomy" but it doesn't feel quite right to me. Any thoughts/advice?

Mrmerlin

could be bent valves,.. slightly,
do a compression check.
if thats good put a bottle of techron into the gas,
also add 6 oz of MMO ,
do short drives to let it work

thepurpleblob

I'm not fond of the "slightly bent valves" theory.....

What's "MMO"?

honerboys

marvel mystery oil - i've never used it so i can't comment on it's abilities

thepurpleblob

Oh Ok... I don't think these things exist here in the UK.

Shark2626

Good! I would never put Marvelous Mystery Oil in my gasoline. Techron on the other hand was designed in the mid ‘80’s to clean carbon from combustion chambers caused by the poor quality gasoline’s that were available back then. Now all EU gasolines are highest quality with additives, like Techron, to prevent carbon buildup. The newer Techron formulas clean the entire fuel system.

Liqui Moly is I think the preferred brand for fuel system cleaners in Europe. BMW guys rave about it. They have several products for this, the Internet will help you select the right one for your car’s needs.

thepurpleblob

Yes indeed. In fact, I am an ex "BMW guy"

Mrmerlin

The MMO will lubricate the fuel pump and the injectors it is completely safe Shark2626 thats why I suggest it.
It works great in all the cars I put it in.
use about 6 OZ for a full tank.
you could also add about 2 oz of Startron to a full tank if your using ethanol in your fuel

Shark2626

Whatever Stan, I’m still not putting oil in my gasoline, even if it’s name is Marvelous Mystery. When Porsche sells the stuff and says it’s a must I might reconsider my position.

Gasoline is supposed to lubricate fuel pumps and injectors. Are you saying MMO sticks to their moving parts and isn’t affected by gasoline?

I bought in to your DeOxit BS, but really that stuff is for guitar pots. It was fun to believe in a fake miracle though! 🤣

Mrmerlin

Shark2626 can you prove any of my suggestions dont work?

NOTE just because you choose not to follow these suggestions,
doesn't mean the suggestion is invalid.

It just means your not interested in finding out what it does for you.

That said I would never offer advice that could damage another owners machine,
nor offer suggestions that could create an unfortunate outcome.

I have no idea about your assertion that the Deoxit 100 is BS .
Maybe you got the Deoxit 5 thats an electric rinse formula.

This is the finest chemical that I have found to enable otherwise damaged electrical connections to be restored to function as designed.
A 928 needs this for the electronics to work at full potential.

I have fixed billions and billions of seat switches and mirror controllers and rheostats with it as well as light contacts, trailer hitch connections you name it.
I have been fixing these cars longer than your shoe laces.

bureau13

Are you under the impression that guitar pots use different electrons than automotive electronics? DeOxit does what it's supposed to do and does it well, it doesn't really matter what the circuit is for.

9two8

Have a look on Amazon UK, couple of years ago that's where I got mine.
Failing that there are a number of specialist UK suppliers on the Web..
Although having read the entire post any advice you received seems a waste of time!

Ken
80 928S
5 Speed
UK
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thepurpleblob

SIgh.....

Petza914

Let's get back on track. OP, how many miles since you changed the plugs? If a few hundred, pull all of them and keep track of which plug came from which cylinder and post pics if them. Let's see if one or two look different from the others. While you're doing this, it's also easy to do a compression test. Pull the coil wire and the fuel pump relay.

Report back with the plug photos and compression numbers.

thepurpleblob

Probably between 50 and 100 miles, just. I'm on it....