Hesitation/surges @2400-2800rpms

[HotWat1]

My '75 2.0 (FI) has developed a hesitataion/surging when I get to the 2400-2800 rpm range. Before and after no problem. Car was serviced (valves etc) a few months ago. Any ideas or tests I need to try to pinpoint it. Thanks.

[Dave at Pelican Parts]

Unplug the throttle position switch. The car should run fine, but idle may be a little funny, and acceleration will really suck. Steady-state cruise should be just fine, though. If the problem goes away when you do this, your TPS is suspect.

Check http://members.rennlist.com/pbanders for a discussion of what the TPS does, in addition to some tests you can do to it and potential fixes.

--DD

[HotWat1]

Unplugged the TPS. Car still "bucks" between the 2200-2800 rpm range. Anything else I should be looking at?
Thanks
Don

[Rrrockhound]

I have had the same problem, with bucking at 2100 rpm and later at about 2700. After unplugging the TPS and not getting improvement, I applied a little suction to the distributor advance and noticed a slight sticking, which made me suspect that grease and whatnot had gummed up the works in the distributor. So I removed the distributor and took out the trigger points and advance plates. I blasted everything clean with brake cleaner, greased everything up lightly, and reassembled it all. I also ditched the Pertronix electronic unit and went back to points and condensor, set the dwell and timing, and replaced the coil while I was at it. Needless to say, the plugs/wires/air filter/cap/rotor/vacuum lines and all that other basic stuff have been recently replaced.

After all this, I still get a very occasional buck every once in a while, but I'd say I have a 90% improvement in driveability. I can drive in city traffic without embarrassment, which was the goal. I suspect the TPS is to blame for the lingering bucking problem, but with the price of new ones, I can live with it. If I were to do a full restoration, I might go all the way and replace the full fuel injection and ignition systems, but all in all, it runs pretty good for a 32 year old car with 124k miles.

[pbanders]

Originally posted by HotWat1
Unplugged the TPS. Car still "bucks" between the 2200-2800 rpm range. Anything else I should be looking at?
Thanks
Don

I'm assuming up front that your mechanicals are all in good condition. Good compression on all four cylinders, good intake manifold vacuum, valves properly adjusted. All sheet metal in place, operating thermostat and flaps. Battery in good condition, charging system in good condition, fuel system with clean filter, proper flow, adjusted to proper pressure. All hoses in good condition with tight sealing, proper routing. All components correct for your year and engine displacement in the ignition and FI system. All FI ground connections clean and tight to bare metal.

Check out the ignition system. New cap, rotor, wires, and plugs. If you haven't converted to an electronic ignition (you should), new points and condenser. Set the dwell with a meter and the timing with a light to spec levels. Remove the points plate, clean and re-grease it. Check the advance and retard cells to verify operation. Check vacuum hoses. Check the PCV valve operation and make sure the hoses are correct. Still bucking? Change the coil. Keep everything you took off to use as a spare if there's no improvement.

Still bucking? Go into the FI system. What condition are the trigger points in? Most I've seen are shot. If they haven't been changed in the past 60K miles, change them. Otherwise, follow through the stuff in my FI articles at my website.

[jk76.914]

I'm going through the same thing with my '76. It started with an annoying-but-I-can-live-with-it-I-suppose shudder at about 2300-3000 RPM part throttle cruise. Idle was fine, as was >3000 RPM or WOT.

Now it's progressed to where it's there at all speeds and it misses even at high speed. I have a thread going at 914club.com detailing everything I've tried, so I won't repeat the details here. You can look for is under my name jk76.914 when the server is back serving again...

Complicating my situation- I put a new cam in. The degradation has occured since about 500 miles after startup of the rebuilt engine. Initially, it ran better than EVER. So it's possible I got a soft cam.... rechecked the valves, and it didn't seem like they were off, though.

Anyway, now I'm about to put new fuel injectors in. It's the only thing I haven't tried. The gas stations around here have switched to gasohol this summer. I'm three tanks into the stuff. The degradation could be crap migrating from the lines to the injectors. Grasping at straws, I know, but I've tried TPS, vacuum leaks (none that I can detect visually or with propane), compression (140-145 all around), timing, MPS calibration (even bought an inductance meter!), etc, etc, etc, etc. When the new injectors go in, I'll be putting a new fine fuel filter in just before the rails....

If you solve it, I'm really interested! I can't even drive it now.

[bd1308]

Have you checked your FI Trigger points Jim?

I have set of good ones if you need them, check the connectors, mine were corroded badly.

b

[jk76.914]

Thanks for the tip. I put new trigger points in at about 300 miles AR (after rebuild), so that was about 650 miles ago. At that time, I rebuilt my distributor to clean and grease the breaker plate. Disty seems fine now too. Rock steady timing mark across the RPM range.

I'm using a Pertronix module- it's almost 20 years old. Someone with the same symptom got relief by going back to points. That's on my list, right after the new injectors which are due in this Thursday.

Thanks again,
Jim