Car is a 1993 base model with a racing beat CBE and a downpipe.

Issue is that when occasionally when cruising at a steady speed/rpm, the car begins to buck and jerk, as if I were feathering the gas. At WOT there are no issues, and I don't notice much difference from the car being cold or hot. Again, this is intermittent.

From searching the forums I've found that the TPS may be at fault. However, my car idles at about 750rpm, which doesn't match the TPS symptoms. Also, I've read about the potential for fuel cut due to overboost. I don't have a boost gauge yet, so I wouldn't be able to verify boost levels. Could the additional airflow due to CBE/DP combo cause that?

I've got the write-ups and how-tos bookmarked, just want a second opinion or some insight into something i may have overlooked, before I go on a weekend long wild goose chase tearing the engine apart. Thanks in advance.

 

What rpm does it happen?

I forget where exactly in the rpm range it was, but the stock ecu had/has a documented hesitation issue. I think it was like 3,500 or something like that...

 

Yeah I don't think it's the hesitation. It happens pretty much anywhere below 4k I'd say. RPM stands at about 2300-2400 at 60 on the highway in 5th and it still bucks. Although, it bucks much harder the lower the gear, which pushes me towards looking at the TPS.

 

Check out post #42 in this thread. Other posts in the thread might be helpful also.

 

Search for checking and adjusting the TPS (Throttle Position Sensor) voltage.
I don't think idling at 750 is a reliable way to check the TPS.

With a PFC you can read the voltage on the commander. Otherwise it is a volt meter, which is pretty hard to do with the throttle body attached.

My FD used to buck like that. Idle was fine.

It bucked when it was stock and bucked when it was converted to single turbo.

I adjusted the TPS - no more bucking.

It can accelerate from about 1000 RPM in 3rd gear with no issues.

 

Imagine a light dimmer switch that gives out at a certain position. To test the tps you must check it for linearity through it's full range. Idle position could be working just fine, but somewhere in between idle and wot there could be a short. You can test this with an ohm meter and some knowledge.

 

Thank you kindly sir, this seems to have completely resolved the issue. First thing that I checked was the battery cable/fuse bracket. There was a wire grounded to the bracket, hidden by the fuse box, that had become seriously loose. I sanded off some of the paint at connection points to expose a conductive surface, and re-bolted it all back in place. Then I changed out the grounding wire between the bulkhead and the hanger. The old cable was in okay condition, and the connections were tight, but again it was "grounded" on non-conductive paint. Sanded that back a little as well and replaced with a new grounding cable. Had a hard time locating the ground behind the A/C, and it was getting late so I left well enough alone, hoping that the other grounds would solve the issue.

The car pulls like a dream now, 1st through 5th throughout the entire powerband. No hesitation (even at 3k), bucking or jerking at all. I hadn't considered the possibility of a ground being at fault, but it makes sense according to the thread you sent me. I remember the mazda 3 having the same issue where the ground connections were not very good due to the non-conductive nature of the paint as tested with a voltmeter. Anyways, car is great now, no complaints, and thanks again!

 

My friends "94 FD also has the same issue. Mods ebay downpipe, no cats, performance muffler. I will also try cleaning the grounds and report back. Thanks

 

You probably just need to bump up your idle speed with the screw, if you're talking about coasting in gear or partial throttle.
It would be good to test your IAC and your TPS, too.

Any more specific information you can provide about your car and exactly when the symptoms occur?