D-Jet Running Issues

Sam Lever

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Hi Chaps and sorry for my second post being a p.i.t.a. technical one........

I spent a lot of last night reading posts on this forum in the hope of tracking down my problem, but think I am going to share it in all its glory here in the hope of some collective help & therapy!

My car is a '72 UK CSL. It is largely standard, though I do run Pertronix in the distributor and also an MSD ignition system, along with an MSD coil. Those mods were done literally years ago and made the car run much more cleanly, with better throttle response and great fuel economy. I run a set of Bosch Super 4 plugs that have been in the car for some time and I have not changed them recently. I do all of the mechanical work on the car myself and always have. I know pretty much where I've been and what I've done, but I am losing my way with this particular problem.

The car starts perfectly, hot or cold and runs for the most part very well. It has plenty of power and a decent top end (standard cam and valves/head). Idle is smooth and consistent, with no surging and it is running around 2% CO, which has been a bit of trial and error, but seems to make for the smoothest idle.

There are two very distinct issues with the running that are causing sleepless nights as I try to trace them:

Accelerate briskly up to, say 80 mph and than back off, as if to maintain that rather than slow down. The car will stutter and stumble, as if I had lifted right off and closed the throttle. Trying then to maintain 75 mph you can feel it stuttering. If you accelerate again it pulls cleanly through that and on to three figures very comfortably indeed, but the 60-80 mph is really tiresome.

Also, in built-up areas trying to maintain 30 mph in traffic involves a lot of very light throttle work and constantly coming off the light throttle and back on again. The car really stumbles on this and has occasionally cut out. Again, a tramp down on the throttle results in a very crisp pick up and it canters away fast. However, in traffic, that is neither possible nor legal and at 20-35 mph it is very uneasy.

This problem has been with me for a while, but has worsened over the past couple of years. So, what I have I done so far?

I suspected the Throttle Position Switch (TPS), so replaced that first a couple of years ago. It made no difference, new or old, just the same.
I have had a spare Manifold Pressure Sensor rebuilt at considerable expense in Germany and the symptoms are ideal to the old one.
I have replaced the ECU with a known used spare - again no change at all.
I have replaced the distributor speed points with a known used spare - no change at all
I have replaced the fuel filter, which was clean enough - no change at all.
I have replaced the main relay with a new one, as I suspected that this might have been cutting,/resonating and causing problems - no change
I wound the fuel pressure up to 32 psi. This did help with a high-speed pinking issue that I had also suffered, but made no difference to the issues noted above.
I have even run the distributor with the vacuum disconnected and plugged, working on the basis that there might be a momentary short or interruption as the plate was rotated within the distributor under varying load and vacuum, but no change.

My conclusion was that this was either related to manifold pressure or throttle position (or both), as these were the only variables that should be relevant in the circumstances where this problem occurs. Knowing that the MAP was now effectively rebuilt and should be beyond suspicion (plus the old one behaves just the same) I focused in on the TPS. I was troubled by the amount of play on the throttle drag-link shaft that drives the TPS and wondered whether under light throttle the top (driven) part of the TPS arm was being allowed to cant over and in some way break its contacts, producing erroneous readings. I separated top and bottom parts of the arm an inserted a washer as a spacer, so as to take up as much of the play between the two as I dared, removing the ability for the drag-link play to influence what was going on - no change.

I'm now running out of ideas as to what to do and what to check. I am thinking about plumbing in a fuel pressure gauge to where the cold-start injector goes and having that gauge in the cabin so that I can see what the pressure does when the symptoms occur. I kind of suspect that the pressure is momentarily dropping, but can't be sure. Either way I figure this will enable me to determine for sure whether this is an injection fault or ignition. If the pressure remains constant throughout then it would point to ignition perhaps, while if it flicks about then it confirms fueling issues?

Having been around the block with this more times than I care to think, I am also suspecting that it might be the wiring loom where it goes to the distributor contact points? This is one of the few legs that is allowed to move around as the engine rocks on its mountings and it could be that a hairline break has formed close to the plug and as I back off this is breaking momentarily????

Does it sound like I've begun to grasp at straws? I think so too!

I'd really appreciate some good ideas guys, as this is really getting me down. To put it into perspective, we went to the Le Mans Classic (4th one in this car) a couple of weeks ago and the trip was uneventful and fun. We cruised at between 80 and 90 all of the way and did about 24 mpg. The hesitation was always there, as per the symptoms above, but it never cut, never failed to start and was its usual joy to drive. This is just spoiling that party and I want to get it sorted.

I thank you all in advance - for reading this far and for any constructive help and guidance you can give to me.

With very best wishes from the UK,

Sam.
 

Sam Lever

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Update:

Had a poke about with the distributor speed trigger points wiring. It all looked OK, but two of the three cables appeared to have been reversed in order in the plug at some point in time (22,12,21 as opposed to 22,21,12 I seem to recall). I swapped them around and put new terminals on the ends, as well as a new rubber boot. I didn't expect the order to make much difference, as they have been that way for the past 19 years at least (!), but I connected it all back up and sure enough the problems are still exactly as before, so that is one more potential culprit eliminated from enquiries.

Any help that any of you can give is going to be greatly appreciated.

Best wishes,

Sam.
 

sfdon

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Get a vacuum reading at idle
Either install a 02 meter or record voltmeter readings from a o2 sensor at idle, cruise, WOT
 

sfdon

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You aware new TPS get plugged upside down and most important- match the connection numbers on the MPS to the connector!
It can be installed backwards
Number to number!
 

Sam Lever

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Thanks sfdon, for your pointers. Can I just clarify a couple of thing please?

I can run a vacuum line into the car and rig a temporary vacuum gauge, so that I can see it while driving. However, my (Euro) car has no O2 sensor and there is no port in the exhaust where I could even fit one to take any readings from it.

You mention the fact that the TPS can be plugged upside down. My plug is shaped in such a way that it can only connect one way. Are you saying that a TPS that has been manufactured subsequent to OEM will have had the connection numbers reversed in its socket relative to the plug in the car's loom? You mention the order of connections on the TPS needing to match the order of connections on the MPS connector. Did you mean the MPS or where you refering to the connector for the TPS? I can check both relative to the wiring diagram, but it seems hard to believe that the car would run so well for most of its range with such a fundamental mess up? Either way I shall check those numbers out toute suite.

Thanks for your help,

Sam.
 

Sam Lever

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OK sfdon, so I've just checked the car:

On the MPS I have (front to rear) 15-10-8-2, which matches the stampings on the connector block on the MPS perfectly.
On the TPS, I have (top to bottom) 17-45-20-9. This matched what the wiring diagram in the blue book says, though the connector block on the TPS itself (and same on my spare) says "17-12/14-20-9". Looking at the wiring diagram there is no wire 14 (unused connection at the ECU) and wire 12 is one of the connections on the trigger points.

I don't think that there is anything wrong here?

Whadayareckon?

Thanks again,

Sam.
 

30csl

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Hi Sam, I would have gone straight to the TPS also - perhaps check the loom from the TPS - get a helper and measure continuity while flexing the loom? Maybe the new one has gone bad - you should be able to test this by seeing a smooth change in resistance through the movement but I know you know this! I would rig up a fuel pressure gauge remotely next - this was how I found a hesitation once that turned out to be cr8p in the tank. If it was a vacuum leak it should be a constant when on the move and also show itself with a hunting idle.

Best of luck!
 

Sam Lever

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I am leaning more and more on the TPS being bad. Investigating this morning getting a new one. I know W & N don't sell them, but let's see.

Cheers Rohan - hope all is well with you.

Sam.
 

30csl

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Hey Sam,
Yes all good thanks.

I would try C3 BMW if you have a manufacturer part number.
 

Sam Lever

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I've got Kirby looking right now :) Funny how we all gravitate to the best places isn't it?
 

sfdon

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With engine off but key on -
Actuate the throttle from the engine bay.
Do you hear the injectors clicking?
 

Sam Lever

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Thanks sfdon. Indeed, I reckon I can hear all 20 clicks as the throttle is opened. However, in the mid range it does sound a bit more like a geiger counter than a metronome...... There seems to be more of a fuzz than a distinct click-click-click.

I have now pretty much confirmed that there are no new TPS's any longer. Bosch doesn't list that part number on its "Automotive Tradition" site and my usual suppliers all show NLA. I am going to revert to my 'old' TPS, which is a good deal newer than the spare that is currently on the car. I think that will be better and it may even be that the hesitation I used to suffer on that one was down to the fuel pressure alone. We'll see soon enough.

I had an idea of a further troubleshooting process. If I rig up a fuel pressure gauge in the cockpit I can see whether there is a drop (or surge, I suppose, but unlikely) in fuel pressure that coincides with the cough. If it doesn't then perhaps I need to be looking at air (play around throttle spindle perhaps) or ignition. If the fuel pressure does indeed drop then I can also rig a lamp in line with the pump's supply out of the relay. If the lamp dies with the fuel pressure drop then it is electrical/electronic, while if the lamp stays on that would imply crud somewhere or a coughing fuel pump/dodgy connection/cracked wire. I have to say that I doubt the fuel pump and its circuitry to have a problem, while the fuel lines are only 5 years old or so and with a new filter, but who knows?

Thanks for the continued stream of ideas.
 

Sam Lever

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Yeah. My last trip to one was awesome, with a nice clean curve and 200.5 BHP. I suspect it would look a bit more like the NASDAQ now :)
 

Sam Lever

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Thanks sfdon - white eraser is a good idea. I shall try that on the newer of my two first and see what happens with that. You mention a rebuilt one - does that mean a new PCB? I notice there is someone in Belgium selling a new PCB for a Bosch D-Jet TPS, but it isn't the right one for me unfortunately :-(
 

Sam Lever

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Not sure they do a TPS? Their site seems only to list a fuel pump for a 3.0si, though I am sure the injectors would be available too
 
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