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Posted by: porsche913b_sp May 16 2011, 01:25 PM

I am trying to help a friend out. The engine hesitates when accelerating in 2nd gear above 3500rpm. Does not occur when cold. Ambient temps > 60F. Did not notice when cold. Check timing ? Injectors ? Dirty air filter ? When this hesitation happens can hear an after burn backfire.

Thanks in advance.

Posted by: zymurgist May 16 2011, 01:41 PM

Sounds like the famous "bucking" problem. The throttle position switch (TPS) is usually to blame for this. There is a replacement circuit board that one of the members here designed and sells that usually fixes this problem.

Posted by: porsche913b_sp May 16 2011, 01:46 PM

Is there any test to confirm this or swap-tronics ?

Posted by: tradisrad May 16 2011, 01:59 PM

clean the TPS and then follow the pelican article on how to set the TPS.

Posted by: Dave_Darling May 16 2011, 02:36 PM

Unplug it to take it out of the circuit. The idle may be a bit ragged or low, and acceleration will suck, but if the bucking is gone you have a pretty good idea that the switch is to blame.

--DD

Posted by: porsche913b_sp May 16 2011, 07:23 PM

Stupid question but the car will run with the TPS unplugged ?
I assume in some default value pre-programmed into the ECU. ?

Thanks Again !!

Posted by: Kraftwerk May 16 2011, 07:27 PM

Dave Darling seems to know what he is talking about. Give it a try and get back to us..

Posted by: ThinAir May 16 2011, 11:03 PM

Testing using Dave Darling's technique pinned down this problem as the TPS on my 1.7 and replacing it confirmed it. I may have the same problem now on my 2.0L, but I can't find any member vendors that offer this. Who does this?

Posted by: Dave_Darling May 17 2011, 01:39 AM

Yes, it runs without the TPS. The TPS is not one of the primary things to figure out mixture, it only changes the mixture under a couple of specific circumstances. One is when the engine is at idle (throttle closed), and the other is when the throttle is being opened up. There is no default value, the lack of a signal is seen as "not at idle, throttle not currently being opened more".

--DD

Posted by: underthetire May 17 2011, 08:41 AM

If i remember correctly, 914rubber had some made.

Posted by: porsche913b_sp Sep 9 2011, 01:40 PM

My friend cracked open the TPS and it did not see any tracks signs, he cleaned it up a bit. But he did not try to drive it with it uplugged. Still tracking issue on TPS ? Verified fuel PSI, and inspected the plugs and they were white/gray according to him. I did not see it. The MPS was bad, so a reman from Atlanta is coming. CHT he forgot to write down the exact PN for his 73 2.0L I have read in the rennlist the 73 2.0 is very finicky about the match set of CHT/ECM/MPS and it says it can cause load performance problems.

Is there anything else he can check. Before saying thats how it is because the components may be mismatched ? Oh he also looked into the possibility of AAR as well and replaced the fuel filter too.

Thoughts ?

Thank you

Posted by: vsg914 Sep 9 2011, 02:35 PM

My 2 cents. The tps is not generaly the culprit in an acceleration miss (unless it is severly misadjusted), but more often it causes the "bucking at cruise" symptom (worn traces). A bad mps will certainly cause the problem you describe, but that would be accompanied by black plugs from the rich condition. I would also suspect the condensor if it has points.

Good luck.

Posted by: vsg914 Sep 9 2011, 02:40 PM

Don't know if he's still got any, but you can go here for new tps boards

http://www.bradrobertsag.com/product/BRAG-906-111A/Replacement-Throttle-Position-Switch-for-73-76-20.html

Posted by: jsayre914 Sep 9 2011, 04:31 PM

I had a hesitation/ bucking in my car.

It turned out the gear at the bottom of the distributor was worn. I swapped out the distributor with another i had in the garage and it ran perfectly.

Posted by: porsche913b_sp Sep 9 2011, 05:18 PM

The idle was fine though, would a mismatched set of components still affect it ? To clarify his MPS did not hold any vacuum. Wouldn't it have more driveabilty problems than the surging with his MPS ?

Posted by: vsg914 Sep 9 2011, 05:34 PM

Ok, Surging you say? At idle? Thats a lean idle condition possibly corrected by the knob on the ecu, but only if the tps is correctly adjusted. An mps with no vacuum will be dumping large amounts of fuel in the cylinders, causing a major stumble on accel, and make it run like dog poop (what ever that is).

Posted by: Tom Sep 9 2011, 06:48 PM

With your description of the plugs ( his ), I would think a lean condition.
Fuel pump, fuel filter, would be my first thing to check.
tom