Bucking
 

Hello,

A strange problem has just cropped up...something I've never seen before.

I've got a 1990 3vze 5-speed 4x4.

It idles perfectly at 800 once it's warm, and accelerates smoothly.

In gear with the clutch out (cruising), as soon as I take my foot off the gas, it decels pretty hard. Giving it a little gas to speed back up, and it bucks some.

At a constant speed in 4th or 5th gear, it feels like I have to always be slightly accelerating or it falls on its face.

It's most noticeable in the lower gears at a constant RPM. It bucks like heck in 1st.

In the last 12 months, I've replaced plugs, wires, distributor, cap, rotor, TPS, O2 sensor, and, not that it matters here, the starter. The motor is a rebuilt put in by a reputable for the PO about 50,000 miles ago. I've driven it about 6,000 miles since all those repairs.

I would think ignition/fuel delivery except it only does it on decel and trying to maintain a constant RPM with my foot feathering the gas. It idles fine all day. No stutter, no miss on engine accel or decel.

No calipers appear stuck or anything. I had that happen in another vehicle once, and it smelled of burning brakes, and pulled hard to one side of the vehicle.

No burning clutch smell, full clutch reservoir. No idiot lights and no codes. I don't think it's clutch slip, because it moves if I get on the pedal, and will pull through the parking brake in both 1st and reverse. RPMS are exactly where they have always been; e.g. ~2000rpms in 4th gear at 40mph.

Gas pedal cable adjustment? Something wonky with the throttle body?

I'm a very experienced manual trans driver, and pretty knowledgeable about cars. Never seen anything like this before.

The best way I can describe the feeling is if you've ever driven an automatic with a bad detent/downshift cable...take your foot off the gas and the thing bleeds speed quickly, but not in a "downshift to slow down" way.

So, help? Please?

Fixed.

I tested fuel pressure, and it was right on the money. I checked the rear brakes to make sure the shoes weren't dragging. Fine there too. Clean air filter, good plugs and no spark leaking from wires. Cap and rotor looked good, so I unplugged the TPS and drove it around the block. Problem gone.

Tested the TPS in the truck, and it failed the E2-IDL test at .77mm.

Yanked the throttle body, and retested. Same results. New TPS and no more problem.

Never seen a TPS go like this. Coulda knocked me over with a feather.

Sweet! Always good to hear when some one wins on a problem like this:D

I'm ever more convinced that the vast majority of power, performance and fuel economy problems in these things are tied to the TPS/ECU system.

The Borg-Warner aftermarket replacements are definitely of lower quality, but at the price it's hard to argue for the more expensive Toyota part that is increasingly difficult to find.

Although rudimentary, the computer has ultimate control (to my understanding at least) of mixture and spark advance. While very good at keeping the vehicle running, even with multiple system faults and failures, it seems rather lousy at keeping the vehicle running WELL with even the smallest hiccup. Worst of all, it's pretty bad at reporting faults.

Yesterday, I pulled the codes (none). Pulled the EFI fuse to reset the computer, unplugged the TPS, and drove it about five miles. No CEL, no stored codes after. But, it ran reasonably well, considering that without the TPS hooked up, the ECU cannot control spark advance; and the engine relies on physics and chemistry alone to keep running in terms of mix.

After my test, I cleared the non-existent codes again, yanked the throttle body and replaced/calibrated the new TPS. Test drove another five miles, pulled the codes (none) and closed the hood.

Long story short, people deride Toyota for bolting on all the extra electronics and computer controller as almost an afterthought for emissions control and mild performance improvements, and I more or less agree. Relying on anything but meticulous DMM-ing per factory service manual and 4Crawler's awesome page probably won't get you anywhere.