engine rebuilt - now sputters from a stop and "bucks"

[kcducttaper]

Long story short, I dropped a crankshaft in my '94 villager (v6, efi, distributor, etc - pretty standard setup for that era), so I got a new crank and threw it in there along with bearings, rings, rebuilt the heads as best I could without going to a machine shop, blah blah blah.... I threw it all back together and now I've got some weird symptoms that I wonder if you guys could help me out with.

- When taking off from a stop, the engine sputters a bit for a second or two. If I stomp on it too hard, I risk killing it. It idles mostly ok at ~800 (slight roughness if I pay attention) and after it gets off the line a bit, it runs fine; it just doesn't like taking off from a stop.

- The check engine light comes on and off sometimes (generally comes on while coasting and goes off under power). The code reads a fuel injector circuit fault (doesn't say which one), but it's firing on all 6. I replaced one injector that was out of ohm spec while I was rebuilding it thinking that was the problem, but no dice.

- When I drive down a slight decline where the engine speed matches the wheel speed, but there's no load, the engine "bucks" a bit (most noticeable around 45mph). It's bouncing between being driven by the wheels and sending power to the wheels, but I can't figure out what's up with that.

- When the engine is warm, the battery seems to pretend that it's going dead (I.E. less torque is getting to the flywheel from the starter), but it's never left me stranded anywhere. It didn't do that before I rebuilt the engine and I've uber-cleaned all of the connections from the battery to the starter with no dice.

It almost sounds like a timing and/or injector problem (possibly MAF, but that doesn't seem quite right), but my timing is set to 15 btdc as specified by the manual (although with 30 btdc, it jumps out of the hole with hardly any sputtering, but the bucking is significantly worse). I've double-checked that my timing belt is on properly and everything is lined up. Gas mileage is about 20-22 hwy, which is pretty typical for this van. I'm not sure what else to check. Maybe I need to tackle the timing a 3rd time and do a full injector job...?

[vgames33]

The bucking sounds like there may be a dead spot in the TPS. The sputtering from a stop could be caused by the TPS as well.

[shorod]

It sounds to me like you have either a vacuum leak, an ignition misfire, or possibly a fuel injector fault. I did run in to a main injector wiring issue on a 1995 Ford Contour, I wonder if your Villager is wired similarly.... On the Contour, the wire that was bad was the primary power wire to the fuel injectors and it disabled the entire engine. However, it was in the same connector bundle and the same type of terminal as each of the 6 injector trigger wires. On the Contour, as soon as I would backprobe the connector to try to get a scope reading the car would start, so if yours is similar it wouldn't surprise me to have an intermittent connector. How do you know that all 6 injectors are firing when the sputtering and bucking happens?

Have you double-checked all intake air tubing between the air filter and throttle body to make sure it's all connected correctly and tight (no lips folded under and letting air in)?

Since you had the heads off there's a good chance you disturbed several brittle vacuum hoses. Have you checked for cracked or broken vacuum hoses and made sure there are no unterminated vacuum ports?

A co-worker of mine had a Mazda Protege that would sputter and run roughly but only under specific conditions (I don't recall if it was under hard acceleration or engine braking). He took it to a shop who pretty quickly discovered that there were cracks in the accordion folds of the intake air tubing. When the engine would shift under those certain conditions the cracks would open up and allow air in after the MAF causing a vacuum leak. As quickly as the shop found this I'm suspecting this wasn't the first time they've seen that. Might be something for you to check as well.

-Rod

[kcducttaper]

I crawled around tracing every injector wire I could find and didn't see anything abnormal. One thing I am curious about is why the primary power wire has 5 volts (with the injector fuse yanked), but drops to 0 when I unplug the computer. I've kinda wondered if my computer is goofy, but I always thought they exploded in a very flamboyant manner when they went bad.

I don't know for sure that they're all firing when it sputters and bucks, but I can't think of any reason why an injector or two would drop out only when the engine speed matches the road speed. If it was a bad connection, it seems like there wouldn't be a very predictable pattern to it.

I'll have to check for leaks when I get a chance. As old as this is, it's possible I cracked something shoving it back in. I squirted out the MAF sensor yesterday already too.

[shorod]

I'm not sure how you are correlating engine speed to road speed. The automatic transmission will not provide significant engine braking when left in Drive regardless of what gear it's in when you let off the gas. But if your engine is at a low RPM when you step back on the throttle and get the bucking and sputtering, the load on the engine is pretty high at that point (low on the torque curve). It's pretty common for ignition misfires to exhibit issues at low engine speed but to seem fine at higher engine speeds.

-Rod

[kcducttaper]

The transmission doesn't provide significant engine braking, but it does provide some even with the torque converter unlocked. The bucking only happens when I'm going slightly downhill and am barely touching the gas only enough to keep it from slowing down due to the ever-so-slight slight engine braking (I.E. engine speed matching the road speed with little to no load). Other than that, it doesn't seem to care about rpm.

[shorod]

Still sounds like either an ignition misfire or a vacuum leak to me.

-Rod