After the third set of old heat exchangers on my '73 1.7 FI, I finally buttoned up exhaust leaks and now the car has started bucking/hesitation. All new ignition parts, timing, TPS exchanged with a new one, no difference. No known vacuum leaks. I can drive the car and it has power, but I have to drive "around" the hesitations. Fuel pump was replaced too. I'd like it to run better while I finish the 2056 carb engine. Ideas?

My guess is the MPS diaphragm has worn out. That can cause bucking problems.

Konrad

Check all grounds and electrical connections around the dizzy and coil.

Check the fuel pressure.....one thing.

Describe the bucking in more detail....

Accel or decel?
High RPM or low?
Cruising or pushing it?

Rich

Mostly during acceleration. It doesn't want to completely go. Sometimes it is worse than others. The car can be driven on the freeway, but it is not running correctly.

Check all fuel delivery items from the filter to the injectors.
What changes did you make before the bucking started?

We only worked on sealing up the intake system. New seals, new big hoses on the intake tubes.

I had that problem twice. I found a leak at the vacuum hose where it connects by cyl#2. The other time it was a bad ground connection at the rear of the block. That one drove the injectors nuts.

Sounds lean.
Check injector seals?
Not visible, but create huge vacuum leaks, and are cheap to replace.

Look for air leaks on the intake.

You might look at the temperature head sensor. Mine went out and I had similar problems on acceleration. Initially I thought it was fuel pressure, dizzy, coil, wiring harness, the ECU It ended up being the temp head sensor which went out. It's a $15 part but can play havoc on the car's performance if it is not working properly. Once it was replaced, all of the bucking problems I experienced went away. My problems started out being worse when the car was cold but then subsided once the car was warmed up. Later on, it happened consistently when the car was accelerating... couldn't even get up a hill. If you do replace it, make sure it is threaded and seated properly.

Did you change to a new TPS or only to a new TPS circiut board? I had a similar problem due to worn out TPS contact;
TPS contact
It is easy to check this, just listen for the 20 injector clicks, if the contact is bad there are many more irregular clicks.

Lars S

Since it's on acceleration, check that your vacuum advance is working. Also check the mechanical advance moving nicely? All springs in place? Timing set right (@3500 with vac advance hoses unhooked and plugged)? Are the FI trigger points clean? Check the resistance; it should be very low.

"New big hoses on the intake tubes."

Is it possible they are collapsing during the acceleration of intake air?

quote name='dknechtly' date='Nov 17 2014, 06:43 AM' post='2111415']
Since it's on acceleration, check that your vacuum advance is working. Also check the mechanical advance moving nicely? All springs in place? Timing set right (@3500 with vac advance hoses unhooked and plugged)? Are the FI trigger points clean? Check the resistance; it should be very low.
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distributor is a good thought since you did mention they went though it twice.
Fuel pressure should help rule out some things.
the MPS i gave you said it helped but did not solve it.
Makes me think fuel.
Kind of frustrating that trying to fix the cars leaks actually happened to make it worse.
Its not a consistent buck, its only under certain load conditions and RPM.
Im not sure its the TPS though.
This is the huge problem i have with all the people that say keep the stock injection, its a huge pain to troubleshoot and often its a shot in the dark.
Carb problems are usually straight forward.

my last TPS was a replacement board and brand new, but was still bucking so i thought for a long time that it had to be something else. But i recently got a really nice clean slightly used original TPS from Bruce Stone and since installing it and getting it set in the right position it does not buck - at all. I would suggest that you try a different one again and make sure it is set correctly.

I had some other similar issues with idle that seemed like it was vac leak, but ended up being a bad CHT as someone here suggested earlier- cheap part test it and replace it. I now have 2 extras because it can cause wierd issues or even keep the car from running-

Have you checked your injection trigger points plate? just a thought as well.

do onet thing at at time and yess it can be a process of elimination.

when i replaced these hoses they were not 100% tight and required hose clamps to make a 100% tight seal on all 4 runners. You can test this by taking a bit of carb spray and spray it at each runner connection while running, it doing this effects the idle, you still have a leak-
easy and quick to test.