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'85 3.2 has bucking acceleration

Have '85 3.2 that will accelerate smoothly if gas is rolled on gently. Mash pedal hard for a quick take off and engine bucks badly---there's no quick acceleration possible without being jerked around.

Engine has new plugs, wires, rebuilt distrib with new cap & rotor, and relatively new fly wheel sensors. Air filt is new. Fuel filt is has around 20k miles. Fuel pump sounds good.

Few things have been suggested to me to date: Vacuum leak. Bad fuel injectors. Worn out cat.

Before diving into this, am looking for opinions.

.
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Current: '80 Silver Targa w /'85 3.2
Prior: '77 Copper 924. '73 Black 914. '74 White Carrera. '79 Silver 930. '79 Black 930. '79 Anthracite 930.

Last edited by Discseven; 05-21-2019 at 04:15 AM.. Reason: Added new cap & rotor
Old 05-20-2019, 06:32 PM
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I had a similar issue on my 86.
Turned out to be a crack in the cap.
Good luck..
Old 05-20-2019, 07:09 PM
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My guess is you clocked in the distributor incorrectly when it was reinstalled. Remove the distributor cap, turn the crank pulley and watch the mark on the pulley as it approaches the TDC mark. Simultaneously watch where the rotor points to on the distributor housing. It should be within a couple of degrees of the mark on the distributor, but if not, the rotor is out of position and probably pointing to another cylinder when the DME tells the coil to fire, and thus hard cut outs when the load is changed.
Old 05-20-2019, 07:23 PM
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Cap is new but worth a check. Thanks FreeForm.

Issue was happening before distrib rebuild Steve---one reason for rebuilding it (advance had frozen up.) While rebuild may have helped to some degree, performance still sux. I believe I got distrib back in correct position but also worth a check. Thank you.
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Current: '80 Silver Targa w /'85 3.2
Prior: '77 Copper 924. '73 Black 914. '74 White Carrera. '79 Silver 930. '79 Black 930. '79 Anthracite 930.
Old 05-21-2019, 04:22 AM
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- A vacuum leak won't really effect you at full throttle. There is little to no vacuum at full throttle so that is when a vacuum leak has the least effect.

- I would also check for things that can be effected by vibration and movement. Check the electrical grounds (at the intake manifold and the transmission). Check all wiring connectors for anything that is loose (every sensor, every harness connector. I'd pay special attention to the large block connector at the front of the engine bay. If it is at all loose feeling strap it together with a tie wrap.) Check the DME for solder connection cracks. (At this point most of them have some sort of vibration crack issues.) Check the insulation of the crank sensors and check their mounting. (I think the hall effect should be stronger at higher RPMs so the sensor gap shouldn't be an issue.)
- One thing that would be a bear to check for is wires that have broken down inside their insulation. The only way I can think of testing is a continuity meter, long test leads, and a bunch of patience.
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Old 05-21-2019, 06:47 AM
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Steve or someone else might know but could a failing Throttle Position Sensor switch not changing fuel and timing be the culprit?
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Old 05-21-2019, 07:08 AM
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Bucking under load is usually caused by a lack of fuel. My first guess would be clogged fuel filter.
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Old 05-21-2019, 07:53 AM
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Check the connection for the injectors on the firewall. The pins on mine were not making a good connection and were causing similar symptoms.
Servicing the connectors so the contacts are tight between the pins and tie wrapping the connector solved it.
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Old 05-21-2019, 08:37 AM
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Check ignition coil. Sounds like you did everything for spark except that. Coil is often overlooked. That plus fuel filters are cheap and easy as mentioned above
Old 05-21-2019, 09:01 AM
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Quote:
Originally Posted by Rodsrsr View Post
Bucking under load is usually caused by a lack of fuel. My first guess would be clogged fuel filter.
20K is a lot for a fuel filter and I would start there too. Crap in there is floating around and when you hammer it all goes in 1 spot. I would think it wound bog instead of buck though.
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Old 05-21-2019, 09:02 AM
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Do Motronic Carreras have the same issues as an L-Jet generation air flow meter wearing out its tracks that it "swipes" for an electrical connection as the barn door opens? John
Old 05-21-2019, 11:05 AM
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Quote:
Originally Posted by 911 Rod View Post
20K is a lot for a fuel filter and I would start there too. Crap in there is floating around and when you hammer it all goes in 1 spot. I would think it wound bog instead of buck though.
I think you're right. Bucking does seem to indicate something electrical. I once had a dme relay fail which caused a terrible bucking when the car reached operating temp. Having an air/fuel meter sure helps troubleshoot these types of issues. Kinda guessing at this point.
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Old 05-21-2019, 11:27 AM
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I'm still leaning towards fuel delivery since they said "accelerate smoothly if gas is rolled on gently." It sounds almost like a severe lean stumble and It brings me back to the throttle position sensor. You can test the TPS with a meter for operation easy enough. Since, as I understand it, the TPS changes fuel mapping for WOT or beyond the tip in point, I would think that if it's not working correctly the issue would show up more under heavy acceleration than under a light acceleration. I could be wrong but it's easy to check.
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Old 05-21-2019, 02:13 PM
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Quote:
Originally Posted by jjeffries View Post
Do Motronic Carreras have the same issues as an L-Jet generation air flow meter wearing out its tracks that it "swipes" for an electrical connection as the barn door opens? John
Yes
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Old 05-22-2019, 05:28 AM
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Quote:
Originally Posted by Rodsrsr View Post
Bucking under load is usually caused by a lack of fuel. My first guess would be clogged fuel filter.
Bucking indicates an ignition problem, a bog indicates a fuel problem. His issue appears to be related to ignition, not fuel.
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Old 05-22-2019, 08:21 AM
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