Discussion
Posting this on PH (It's already on a 928 owners-club list, but I'm getting nowhere).
Basically I have an '81 928S with the K-Jet mechanical fuel injection. The car starts from cold, or from hot (if only stopped a short while). However if she's been allowed to cool (for an hour or two), she'll refuse to start (just cranks and cranks and cranks). Let her cool more (ie overnight) and she'll start fine.
I've been investigating this since she did this to me last tueday night (Mrs S and I had gone out for the evening and needed the car to get home!). Eventually after bump-starting the car, I was able to get her running well enough to go home. When I did the bump-start, she was very reluctant to spring to life.
I'd struggled to get this to happen when I take the car to/from work (it appear that the fifteen mile commute isn't long enough to trigger the symptoms), but today I took a longer route (and sitting in a queue because of a crash helped too!!).
So I've now been able to rule out a few things: She has got spark when she cranks. I've bypassed the fuel-pump relay and made it run all of the time (no joy), I pulled the connector plug off the cold-start injector (incase this was flodding her) but no change. Pulled the temperature sensor and left the connector open-circuit, no joy.
Only simple thing left to try is to short-circuit the temperature sensor to see if that will cause the car to think it needs the cold-start (not sure if open-cct or short-cct is "cold"
Any suggestions welcome.
Basically I have an '81 928S with the K-Jet mechanical fuel injection. The car starts from cold, or from hot (if only stopped a short while). However if she's been allowed to cool (for an hour or two), she'll refuse to start (just cranks and cranks and cranks). Let her cool more (ie overnight) and she'll start fine.
I've been investigating this since she did this to me last tueday night (Mrs S and I had gone out for the evening and needed the car to get home!). Eventually after bump-starting the car, I was able to get her running well enough to go home. When I did the bump-start, she was very reluctant to spring to life.
I'd struggled to get this to happen when I take the car to/from work (it appear that the fifteen mile commute isn't long enough to trigger the symptoms), but today I took a longer route (and sitting in a queue because of a crash helped too!!).
So I've now been able to rule out a few things: She has got spark when she cranks. I've bypassed the fuel-pump relay and made it run all of the time (no joy), I pulled the connector plug off the cold-start injector (incase this was flodding her) but no change. Pulled the temperature sensor and left the connector open-circuit, no joy.
Only simple thing left to try is to short-circuit the temperature sensor to see if that will cause the car to think it needs the cold-start (not sure if open-cct or short-cct is "cold"
Any suggestions welcome.
it sounds like you are losing fuel pressure when the car is switched off,this becomes a problem when HOT because you get vapour locks it the injector lines(fuel perculation)the injector injects vapour only(no good)this was a problem on many fords of that era,once the car as been cranked for some time then the vapour lock is purged and fuel is injected,the car will generally run rough until all the injectors have purged,things to check are fuel pressure accumulator,fuel pump non return valve,and excess dribble from the injectors when you switch off,it will save you a lot of hassle if you can get the car to someone that as knowledge of the BOSCH K jetronic system,have primary & secondary pressures checked,if they know there stuff they will also know this is a common fault of this system otherwise it can get mighty expensive to start changing any or all of the above without finding what part/s are faulty,good luck.
Edited by That Daddy on Monday 29th January 13:14
Sounds like it could be related to the warm up regulator that controls fuel pressure from cold. There could still be enough heat from the engine after a couple of hours to 'confuse' it.
The accumulator will only hold the pressure for 20-30 minutes.
The fuel pump relay is also the power supply switch for the warm up regulator and the additional air device. They both have heater and bimetallic switches inside.
By constantly bypassing the relay the two parts become continuously powered and will not be operating according to the real conditions.
Normally these two are only energised when the fuel pump relay is on (when there are ignition sparks)
I pulled the connector plug off the cold-start injector (incase this was flodding her) but no change. Pulled the temperature sensor and left the connector open-circuit, no joy.
I assume you mean the thermotime switch?
This is grounded when cold. It only activates the cold start injector for a few seconds when cold.
The accumulator will only hold the pressure for 20-30 minutes.
sheepy said:
I've bypassed the fuel-pump relay and made it run all of the time (no joy),
The fuel pump relay is also the power supply switch for the warm up regulator and the additional air device. They both have heater and bimetallic switches inside.
By constantly bypassing the relay the two parts become continuously powered and will not be operating according to the real conditions.
Normally these two are only energised when the fuel pump relay is on (when there are ignition sparks)
sheepy said:
I pulled the connector plug off the cold-start injector (incase this was flodding her) but no change. Pulled the temperature sensor and left the connector open-circuit, no joy.
I assume you mean the thermotime switch?
This is grounded when cold. It only activates the cold start injector for a few seconds when cold.
Le tvr,thats not symptoms of a warm up regulator that i have ever come across,it is not maintaining pressure long enough to resist heat soak/vapour lock,if its started quickly when hot shortly after switching off it does not have long enough for the vapour locks to occur hence no problem,thats why when it cools it starts.as i said get it checked before you start random part changing,its not even an expensive diagnostic test.Warm up reg only sets the control pressure for fuel enrichment(ie,cold starting&high speed enrichment) not maintaining pressure or pressure loss.
I've been pointed towards two possibilities: The air-flow sensor and a leak in the fuel system causing air to enter the pipe (eg a leaking injector).
The air-flow sensor in the K-jet for the 928 apparently can get jammed in a way that allows air in (so the engine cranks) but doesn't move the plate to allow fuel to be pumped through. hence it cranks and cranks etc.
Leak in the fuel system will only be detected by using a pressure gauge and looking for loss beyond spec.
The air-flow sensor in the K-jet for the 928 apparently can get jammed in a way that allows air in (so the engine cranks) but doesn't move the plate to allow fuel to be pumped through. hence it cranks and cranks etc.
Leak in the fuel system will only be detected by using a pressure gauge and looking for loss beyond spec.
It sounds reasonably major. In my experience with my 2 Mercs, K-jet is a pretty solid and reliable system to get the engine actually running, although if poorly then starting and idling can be less than ideal.
Mercedes' implementation is in such a way that (so I believe) the car will start and run without the Jetronic ECU even plugged in. I imagine it will run about 90% perfectly this way too. The electrics just provide trimming features based on the sensors it reads, such as engine temperature, air temperature, and control the devices such as idle control valves, cold start valve (which in my car only operates below a ridiculously cold air temperature anyway, so is unlikely to be your problem) and EHA (the rather important electro hydraulic actuator that adjusts/increases fuel flow, richening the mixture when cold). The car should still start and run without any of this if your 928 is done like Mercedes' system. You could try removing the ECU during one of its moments where it's working perfectly, just to see how much worse it runs without it.
That's what I have taught myself in the last few years. So I think if you are getting a problem with non running I think it's likely to be something more major than a failed air temperature sensor or anything.
Also on Mercedes' system, the jetronic ECU is clever and if everything is functioning normally, and then one moment it sees a sporadic reading on a sensor (i.e. because the sensor is dodgy or has a dodgy connection), it ignores that sensor as it is obviously not right and uses 'default' values. This means that if while the engine is running you try unplugging some of your sensors, expecting to see a change in running behaviour, you won't! So it's not a good test of whether a sensor is working or not. However if you turn the engine off and disconnect sensors then restart, it should make a difference as the ECU has lost its reference to work from...
Good luck. I don't envy you!
Mercedes' implementation is in such a way that (so I believe) the car will start and run without the Jetronic ECU even plugged in. I imagine it will run about 90% perfectly this way too. The electrics just provide trimming features based on the sensors it reads, such as engine temperature, air temperature, and control the devices such as idle control valves, cold start valve (which in my car only operates below a ridiculously cold air temperature anyway, so is unlikely to be your problem) and EHA (the rather important electro hydraulic actuator that adjusts/increases fuel flow, richening the mixture when cold). The car should still start and run without any of this if your 928 is done like Mercedes' system. You could try removing the ECU during one of its moments where it's working perfectly, just to see how much worse it runs without it.
That's what I have taught myself in the last few years. So I think if you are getting a problem with non running I think it's likely to be something more major than a failed air temperature sensor or anything.
Also on Mercedes' system, the jetronic ECU is clever and if everything is functioning normally, and then one moment it sees a sporadic reading on a sensor (i.e. because the sensor is dodgy or has a dodgy connection), it ignores that sensor as it is obviously not right and uses 'default' values. This means that if while the engine is running you try unplugging some of your sensors, expecting to see a change in running behaviour, you won't! So it's not a good test of whether a sensor is working or not. However if you turn the engine off and disconnect sensors then restart, it should make a difference as the ECU has lost its reference to work from...
Good luck. I don't envy you!
That Daddy said:
it sounds like you are losing fuel pressure when the car is switched off,this becomes a problem when HOT because you get vapour locks it the injector lines(fuel perculation)the injector injects vapour only(no good)this was a problem on many fords of that era,once the car as been cranked for some time then the vapour lock is purged and fuel is injected,the car will generally run rough until all the injectors have purged,things to check are fuel pressure accumulator,fuel pump non return valve,and excess dribble from the injectors when you switch off,it will save you a lot of hassle if you can get the car to someone that as knowledge of the BOSCH K jetronic system,have primary & secondary pressures checked,if they know there stuff they will also know this is a common fault of this system otherwise it can get mighty expensive to start changing any or all of the above without finding what part/s are faulty,good luck.
Edited by That Daddy on Monday 29th January 13:14
this is spot on - big granada and capri problem in the eighties - I'm too young obviously but my Dad told me all about it
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff