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Posted by: prrt1 Aug 21 2022, 03:09 PM

Need help please! I have a certain car (someday it will be a Porsche) with D-jet, rebuilt manifold air pressure sensor, new injectors, new fuel pressure regulator, and a new PC board in the Throttle Position Switch. While cruising, I get a hesitation (pretty severe) that feels like some is flipping a switch off/on. Wondering if anyone can give me an idea what might be wrong? I hope my terminology is OK..and I searched but no results...
Thanks for any help.

Posted by: mgphoto Aug 21 2022, 03:20 PM

Clean the trigger points in the base of the distributor.

Posted by: StarBear Aug 21 2022, 03:27 PM

Or maybe water/moisture in gas. Happens (to me) this time of year with partial tank of gas and high humidity then condensation overnight. Fill up tank with a quality gas.

Posted by: brant Aug 21 2022, 03:41 PM

Maybe double check the TPS position with an ohm

Posted by: BeatNavy Aug 21 2022, 03:44 PM

This one has a lot of potential causes. It could be fuel delivery related or ignition related.

On the fuel side:
Dirty fuel tank "sock" filter
Clogged fuel filter
Problem with fuel pump
Loose injector grounds (check the 3 prongs at back of engine case)
Loose injector connectors
Clogged injectors (not likely in your case with new injectors)
Bad or misadjusted trigger points
Bad, dirty, improperly adjusted TPS

Ignition side:
Loose spark plug wires
Something wrong with or in distributor -- check ground strap, check condition of dizzy cap and rotor, make sure points are in good shape and adjusted properly

You can eliminate the TPS as a variable by unplugging the connector at the TPS. Car won't accelerate as well, but it will run ok and you can see if the problem persists.

Try the easy things first: check injector connections, injector grounds, spark plug wires, and condition of dizzy components.

There are a LOT of threads on this. Try searching again "bucking while cruising" http://cse.google.com/cse?cx=002408345898193228772:ajvfxschcgc

Posted by: prrt1 Aug 21 2022, 04:31 PM

Excellent suggestions everyone...thanks very much!

Posted by: emerygt350 Aug 21 2022, 05:06 PM

Is it only cruise and only when you are just barely keeping the throttle open?

And is it the stock distributor?

Posted by: 930cabman Aug 21 2022, 05:09 PM

QUOTE(BeatNavy @ Aug 21 2022, 03:44 PM)

Posted by: prrt1 Aug 22 2022, 03:55 PM

Original dizzy, ca. 1975. I have suspected the trigger points, took them out, cleaned, and seem to be OK per Dr. D-jet specs, but I think the rub blocks are at their limit. New fuel filter, checked the pump inlet and it seems to be OK. Crane ignition (no points.)
The bucking happens at about 2000 rpm, 50 mph or so, but it has happened on acceleration also.
I'm getting much good input from everyone, will try a few things. I will reveal the car is a 1974 BMW 3.0CS into which I installed D-jet from a '75 coupe. Joined this group because, well, Porsche guys are as fanatical as BMW guys and hope there is some knowledge I can pick up...
Thanks.

Posted by: DRPHIL914 Aug 22 2022, 04:31 PM

QUOTE(brant @ Aug 21 2022, 05:41 PM)

Posted by: emerygt350 Aug 22 2022, 04:55 PM

He has a new board in the tps. I am starting to think you really need to adjust it by hand in some cases. I was having to do this recently as well. By that I mean setting it 'correctly' on the bench doesn't always cut it.

Posted by: prrt1 Aug 3 2023, 02:27 PM

Welp, a year later and I still have hesitation, sometimes severe (as though the engine momentarily shuts off, it bucks, but doesn’t stall), I tested everything as suggested herein. I think I have narrowed the problem down to the TPS: My car runs OK with the TPS disconnected, but as soon as I plug it in, I get burps and eventually more severe hesitation. I have fiddled with the TPS (it has a PC board only a couple of years old) to no end. I’m thinking it might be a bad wire to the ECU, but before I rewire it I was wondering… if everything is OK without the TPS, what do I need it for? Thanks for all your help.

Posted by: brant Aug 3 2023, 02:42 PM

car normally runs without TPS
but does not run well
TPS is important throttle information for the "computer"
adding additional fuel...

you need it.
did you try to set it correctly with an OHM meter?
did you try to clean the board inside of the TPS housing
then next replace the board if you have done the above
and reset it correctly to baseline with an Ohm meter

Posted by: windforfun Aug 3 2023, 02:47 PM

Did you align the new TPS board exactly like the old one? Are the two boards geometrically identical?

Posted by: emerygt350 Aug 3 2023, 08:54 PM

Are you sure this isn't a lean miss?

Posted by: TX914 Aug 4 2023, 05:43 AM

May want to check CHT sensor and CHT lead wire for wear/corrosion. Assuming lead is good, a quick test for CHT is with engine warm, remove lead and ground it.

Posted by: Tom_in_NH Aug 4 2023, 12:49 PM

Classic symptoms of a D-Jet TPS issue. Very detailed investigation here:
https://sw-em.com/bosch_d-jetronic_TPS_investigation.htm

Posted by: windforfun Aug 4 2023, 01:09 PM

I had the same problem some years back. I replaced the TPS with a new one & aligned it just like the old one (which was badly worn). The problem was solved.

Posted by: prrt1 Oct 12 2023, 03:25 PM

I have found a “solution” to my cruising hesitation and severe bucking problem in my D-jet equipped BMW (I’m writing to 914 World because you guys set me in the right direction. D-jet BMWs are few and far between in our world.)
Anyway, the posts in 914 World gave excellent information (especially Tom in NH) about this problem but no one seems to have identified how to fix it.
My solution was to cut wire #9 between the TPS and the ECU. Wire #9 connects one of the sawtooth traces in the TPS. I found this by chance, but the severe bucking and cruising hesitation is gone, except for a very minor stumble when shifting from 1st to 2nd. Also the hard starting-when-warm problem has disappeared. I don’t know why this fixed the problems, but I running with it for now.
I want to thank 914 World for all the help they have provided in my 2-year struggle to fix this problem.

Posted by: rjames Oct 12 2023, 03:37 PM

QUOTE(prrt1 @ Aug 22 2022, 02:55 PM)

Posted by: brant Oct 12 2023, 03:44 PM

But he also s water cooled so the motor rpm’s are not important for cooling

Posted by: rjames Oct 12 2023, 03:51 PM

True, but it's lugging the engine quite a bit if he's cruising at 2K RPM which would increase temps. Should still be 3K or over. Any increase in throttle at 2k RPMs while doing 50 MPH would induce some hesitation.

Posted by: prrt1 Oct 13 2023, 01:40 PM

I know cutting wires is not a good idea. It is one of the two wires feeding the comb contacts in the TPS (wire 9 in my BMW) so it’s half of the “accelerator pump” now out of the circuit. The huge problem of severe bucking ( occurring when upshifting between 1st and 2nd) and the minor hiccups when cruising (that’s about 2-2.5K rpm) have both disappeared with the wire cut. Very little decrease in acceleration ability. I know it’s weird, but it works. I’ve tried adjusting the TPS (new PC board) per D-jetronic and that didn’t work. I figure most of the info the ECU needs to know about fueling the engine is coming from the MAP, the TPS is sort of a fine-tune, and the cut wire may be eroding my fuel mileage, but that isn’t a real concern. After a couple hundred miles or sooner I’ll pull the plugs and check for mixture problems.