Hello, I had my 73 2.0l out for its maiden voyage this year and the car was running great. On the way home I stopped at a light and when I took off the car was bucking horribly. I investigated and found the bolt under the plenum that grounds the wires for the TPS and, I think, the injectors had come out. I re-installed it checked the grounds, checked continuity between the ecu and the TPS and all is good. I checked the continuity of the TPS as the throttle is opened and closed and it appears that there are about three position that are open circuit. They are small gaps though, barely having to move the throttle to get continuity again. It this normal? Should there be 'flat' spots as I move the throttle or should there be 100% continuity all the time? I did remove the TPS and have a look and it looks fine. Could an arcing ground have caused the TPS to fail?

Thanks in advance!

Jon h.

"Greater FI minds" than me will have more / better suggestions, but it's hard to know what exactly you mean when you say you checked continuity between TPS and ECU. How (which pin on the ECU to where on the TPS)? When you say you inspected the TPS, did you look at the circuit board and inspect the traces to see if they are worn or damaged?

Here's a reference: http://www.914world.com/bbs2/index.php?sho...=95583&st=0

I believe one way to check normal function is to put the ignition on (but engine off, obviously) and with a very quiet garage slowly move the throttle body from idle to WOT. You should hear about 20 tiny "clicks" in regular succession that are the injectors firing as it goes from trace to trace through the whole progression. If those don't happen in regular manner (or if you don't hear the right number), your TPS is not correctly sensing the movement of the throttle and you will get that trademark bucking. Could be worn traces or damaged contact arms.

...check the connection from the harness to the CHT (cylinder head temperature sensor).
bad sensor or connection could cause bucking.

or at what RPMs are you getting the bucking?
consistent at certain RPMs....or thru all ranges?

I checked the TPS using pins 9 and 12 on the ecu and moving the throttle as per the testing procedure I found here on the forum. I also tested between pin 12 and 17 and it checks out. But yeah, I don't here those clicks anymore and I do know what to listen for. When I get home I'll dig I to the thread you linked. I did remove the TPS and inspect the traces. There was nothing obvious that would indicate a failed trace.

Thanks for the reply'

Jon h.

The bucking is while at cruising speed, 1800 or so, if my tac is accurate. I still have power off the line. I'll check the cht tonight.

Thanks. Jon h

Ok, so 17 is the idle pin and you should get continuity to ground only when the throttle is closed. 12 and 21/22 are the trigger points. I need to find that testing procedure. I'm not sure I've seen it here.

If you don't hear the clicks and you otherwise think the TPS isn't damaged, then I guess I would still suspect a FI wiring harness ground issue (you said you reconnected the grounds - did you check continuity, say from that common ground point back to the negative terminal of battery?), a bad harness connection to the ECU, one or more injector connectors not seated properly.... what else?

I checked the injectors and all were seated , I'll double check and grounds. I guess I should try and find out why I don't hear the clicks, that I do know isn't happening. I was focusing on the TPS because the grounding issue and I thought it was a simple fix. .......Sigh.

if cruising then probably not CHT. if off the line it's all jerky, then it could have been...

Dirty advance plates

I should have seen that coming

Weirdest behavior can be traced back to those pesky advance plates.

If the FI is checking out, and the bucking is only during acceleration from stop, could this be clutch chatter? No, I'm not a mechanic, so don't go too hard on me.

but no....not during acceleration....why I asked....Jon H. said, "The bucking is while at cruising speed"

To Rich's point, here's a brief example of someone cleaning their dizzy to make sure the advance plates move freely:

http://www.914world.com/bbs2/index.php?showtopic=94299

Whether or not that's your specific problem now, if it's never been done on your car (as far as you know), it's good maintenance to do.

Not clutch chatter, i know what thats all about.

I just checked for the TPS clicks again and now i hear them, about 13 or so (weird, they weren't there before) . I just removed the TPS plug and went for a drive, no difference, it still bucks. When i first started looking into this a few weeks ago I did notice that the distributer was oily inside so I removed it and pulled it apart to clean it but I didn't pull the trigger points out. I didn't want to mess with setting them up. I also didn't check to see if the advance plates were in working order. They looked fine, but what do i know. Im going to test the entire FI harness and make sure all is good.

Regards'
Jon h.

I just checked mine in the garage (D-Jet, 2.0): 20 clicks. Are you sure you only heard 13? To me that sounds like there is a trace problem on your TPS board. That's fairly easy to temporarily fix (the pencil eraser and/or slightly bending the contact arms) and permanently fix with a new board.

Have fun but most likely you will chase your tail for a while until you do a proper tune, clean the advance plates and re time the motor

Ockams razor

Well, after double checking the wiring harness I discovered that the ground that had first come unbolted under the plenum had done it again. The threads in the engine block became stripped. I moved the grounding point and all is good! Thanks for the help. I'm glad I asked because it got me to re-check and re-think my thought processes.


Regards'

Jon h.

Now if only I can get my other car off the rotisserie!

Wish I had a dollar for every time I have heard of this symptom. When I had this happen I would have bet it all on the TPS which I replaced with NOS genuine Bosch. Turned out to be distributor in my case.