Hi I am trying to hunt down warm/hot start problem on my 1984 928S. It is ROW-Sweden 4.7/228kW non-cat LH-Jetronic with 5-speed.
It starts fine cold and runs and idles no problem. When I try to start it after even short drive cylinders sporadically fire but it does not pick up RPM to idle on its own. I have to crack open the throttle to rev it to about 2000 RPM and then gently let RPM fall back to idle. Sometimes I can manage it on first try but with hot engine it take repeated cranking and throttle finessing to get idle to stabilize. Once it idles everything seems fine and I can drive it. It does not stall on its own but It is quite easy to stall on clutch. I don't have experience with other 928s or LH-Jetronic cars so it could be normal.

Car has the problem since I bought it about a year ago. They changed classic car registration process in my country so it took forever to register and I could not drive it and "debug".

Thinks I already checked/done:
- New Bosch Fuel pump. Old one was noisy after ~30 minutes of running. It was correct Bosch unit made in 2010. Change had no effect.
- Fuel regulators hold stable 2bar at idle and rail stays pressurized with engine off as WSM states. Fuel damper does not leak. They are all Bosch "Made in Germany" so almost certainly replacement units. Originals would be "West Germany".
- Temp Sensor II measured 2.2kOhm to GND on both pins as it should for 15-30C range per WSM.
- MAF was rebuild by JDSPorsche about 5 years ago. Pin 6 to 3 has 307Ohm, Pin 5 to 3 has ~4Ohm. First value is in spec but it is very wide 0-1000Ohm in WSM.

Thinks I suspect:
- Large O-ring seal between throttle body and MAF felt a bit loose. It could let in some unmeasured air but there was no hissing at idle. Ordered new o-rings just to make sure.
- Aux air valve. I have not checked it yet but car starts fine at 0C/30F or 30C/85F as long as engine is cold.
- Could it be caused by very old injectors or EZK/ignition? I would expect rough running with both it it revs nicely on the road.
- When I crack open the fuel rail on cold engine almost no fuel leaks out. Is that normal? On K-Jetronic it would mean bad fuel pressure accumulators. K-Jet usually dumps 50cc of fuel on you even after pressure is gone from the fuel system.

Any suggestions are super welcome. I am used to K-Jetronic injection on old Mercs but LH-Jetronic and Porsche are new things for me. BTW the way Porsche built 928s is absolutely nuts ... in a good way.

Thx. Dave

 

first put a bottle of techron into the fuel tank,
and also 6 oz of MMO,
then do short drives

 

Does your car have a cold start/ninth injector? I had similar symptoms on my 1980 L-jet car recently, and a leaking cold start injector was the issue. The car also held fuel pressure much longer than the book specification, which I still don't understand how when that injector was leaking.

 

Hi Andrew, I have a likewise issue with my 928S 1980 kjet. I always need to start the car with full throttle open. And when I connect my cold start injector, it is very hard to start the car.
How do you notice that the cold start injector is leaking ? Once my car is started it is running rough (engine shakes a bit instead of being still)
It also vibrates a lot during acceleration with lack of power.
Once I have driven the car for at least an hour, the problem is gone.
All spark plugs have nice grey color.

 

I'd say temp sensor or leaking injector(s) or fuel pressure regulator or dampener. Pull the vacuum lines off those and see if there is fuel or the smell of fuel in the vacuum hoses.

If you have a cold start injector, disconnect the electrical plug from it after you drive it and then try the warm start - better, worse, or no difference?

 

LH cars do not have a cold start injector. My first guess is leaking injector(s). There should be some residual pressure in the rail on a cold engine, and it sounds like the injectors are leaking enough fuel to flood the engine with too much fuel on hot starts.

Do what Stan suggests as first course of action - this may free up any sticking injectors.

My 84 Euro LH would do this occasionally. Eventually I replaced the injectors and have not had it happen since.

 

I found the leaking cold start injector through elimination of everything else. Replaced the damper and fuel pressure regulators, disconnected the temp II sensor, etc. Finally the cold start injector was the only culprit left. Unplugging it won't help solve the issue if it's leaking. Replacing or removal along with a plug for the banjo connection are the only ways to eliminate the possibility.

 

Checked pressure regulators and damper today and they don't leak fuel out of vacuum ports.
But I think I found my culprit no.1. Injectors look like original tan colored 252 Bosch pieces with quite hard rubber gaskets under them. I could clearly smell gas from intake this afternoon because I left the car without MAF and air box over night.
I will rip the injectors out and have them tested but if they are original they are almost certainly toast. Probably good preventative maintenance if they are that old and not designed for E10.

Now where to find 928 606 119 00 / Bosch 0 280 150 252 injector replacements in Europe. Looks like Bosch gave up on this style of injector in 2017. Do you know what brand are the yellow ones that 928srus.com sells ... not sure if pre-order means wait a year or month. Rosepassion.com seems to have some but they want 150EUR for one.

 

Andrew, I checked my temp sensor and it gave the correct resistance and works as it should be. For the coldstart injector, perhaps I can just replace it by a closed banjo connection. In summer, the coldstart injector is not necessary.
Since I have a CIS kjet system, I don’t have any fuel damper or pressure regulators. It is regulated via my FD, WUR and fuel accumulator which are all overhauled professionally

 

@Dave383 : Here's a link I've had open for a while; there's a lot of ways to look for a suitable injector - https://cars245.com/en/catalog/order...7045.injector/

As a reference, 19lb injectors work out to about 209 cc/min or 143 g/min (heptane, using calculations I found online).

Cheers

 

you can use MB injectors,
they are not as long but will work fine.
hardest part is getting them out ,
first blow out the sand in each injector collar,
then start soaking them in PB blaster for a few days.

 

Sound like a lot of fun.

 

The OP's car (Dave282) is an 84 Euro with LH-Jet. It has no cold start injector and uses 24lb high impedance injectors (not the same as US L-Jet cars, which use low impedance). I think Stan may have been confusing the mention of a K-Jet car in this thread when discussing using Mercedes injectors - AFIK, that applies only to the K-Jet (CIS) cars with mechanical injectors.

 

I don't know if it's your jam to look for stuff like this but, if you look through the link I sent above, you can find things like this, currently at under $25 per injector: https://cars245.com/en/item/bosch-0280155930-injector/

That particular part comes out being a 23 lb injector (251 cc/min, 172 g/min), but is not too far off from a 24 lb injector (264 cc/min, 181 g/min). There may be more suitable injectors available that match better. My point is that I think you can find something suitable as a replacement for not too much money, and with the Bosch part number in hand, you can probably hunt down a local source.

Cheers

 

I thought we were talking about a CIS engine