TBI Issues, please help!!!

[Josh_Path2346]

It seems that no matter what I do I can never get this right. I've rebuilt several different throttle bodies using components from several different vehicles and none of them seem to work, aside from the original one that came with the car, of which I have never opened. I've tried different methods of cleaning each part of the throttle body including submerging in a ultrasonic cleaner and then reassembling with a gasket kit and following the instructions to the letter. 

 

Whenever I put on a different throttle body it acts like it's not getting any signal to the mass air flow sensor, the idle surges up and down and it sounds like a massive vacuum leak. This is a problem for which I brought it back to the local dealer in 2019 and had them fix by running a wire directly from the MAF to the ECU. Recently I decided to fix this myself as I discovered what they had done and was not very pleased. I tore apart the entire wiring harness and replaced several shielded cables and installed brand new, watertight connectors on everything related to the engine.

 

Since then it had been running great for about 2 weeks. I took it through a car wash and noticed that my idle was below 500, something that I tuned myself, and the engine usually hovered around 850 while sitting at idle in park.

 

I decided I've had enough, and since I had a spare intake, I removed all of the vacuum ports and plugged them with brass plugs wrapped with plumbers tape to get an airtight seal. Since replacing the throttle body it is been doing exactly what it did 5 years ago, which is a surging idle that is between 1,500 and 2,500 RPM, vacuum leak noise, but it isn't running rough. Their fix was, as mentioned above, just a stopgap to get me out of their shop. 

 

I guess my question is, is there something I didn't do right when rebuilding my throttle body? Is there something I'm missing as far as deals or gaskets? Does anyone have a spare TBI unit I can buy that might help solve this problem? If I keep running into trouble I'm going to just buy the Weber carb kit and be done with it.

 

The instructions I followed can be found here: https://drive.google.com/file/d/1purnxdLGXwa3QRe0OGG8C_oKFBT-y7iQ/view?usp=drivesdk

 

And the rebuild kit I used is here: 

https://www.rockauto.com/en/catalog/nissan,1988,pathfinder,3.0l+v6,1211791,fuel+&+air,throttle+body+repair+kit,12939

[Slartibartfast]

I haven't worked with the TBI, but I've read that they tend to run poorly when the air cleaner is off. I guess it smooths airflow over the MAF or something. Wouldn't explain the RPM dropping at the car wash, but if you're testing it with the air cleaner off, throw it on and see if it clears up any.

 

Idling low at the car wash makes me suspect something got wet. If it was shaky, I'd be looking at ignition components (wires/cap/rotor). Otherwise, could be electrical.

 

IMO your best bet at this point would be hooking it up to a smoke machine. That should find your vacuum leak pretty quick. Could be a rolled O ring, throttle shaft seals (the instructions you linked say not to dunk those in cleaner, not sure why not), something along those lines. I would also check that the brake booster, PCV, and EGR (if that's still around) are in good shape, in case it's not the throttle body that's acting up.

 

I don't have a source for the '88 service manual, but on the off chance you haven't found it already, here's the '89. EF&EC section might help you track it down, or at least understand how the system is supposed to work, which might point you in the right direction. '87 had a different code table and some other weirdness, so if yours is closer to '87 than '89, and the codes you pull don't make sense (44 was "no malfunction" on the '87 ECU, replaced by 55 on later years), let me know and I'll dig out my paper copy of the '87.

[Josh_Path2346]

Before all of these issues it would run just fine with or without the air cleaner. After the car wash incident it got a new cap, rotor, and wires. I also cleaned the plugs just to be sure. Not sure if I could build a smoke machine but that would be something I might have to take to a shop and I doubt anyone around here could figure it out. The EGR has since been deleted and is blocked off by a gasket and metal plate that I made, which I suppose I could check and make sure. I just finished rebuilding my other throttle body, and plan to put it on tomorrow when I get a chance. I have the entire FSM in my Google drive, and since this seems to be a vacuum problem with a code 55 on the ECU (1988), there's no way I can use the OBD system. I have several MAFs that I tried with the rebuilt TBI but neither of them changed anything. When I rebuilt the new TBI unit, I took it all the way down to just the shell, meaning I even removed the throttle bearings, seals, and butterfly shaft and then put them all back after it was clean. The only thing I could have found was that the idle up solenoid with the blue shrink wrap on it has exposed wires at the end. However after the car wash incident it did seem to clear up, but a whistling noise and persistent lack of power developed which led me to take this course of action.

[Slartibartfast]

Good, sounds like you know what you're working with. Too bad the computer has no idea what's up, but yeah, that's pretty typical for old EFI.

 

There are a few designs on Youtube for homemade smoke machines. The cheapest I've seen was a hand-operated transfer pump with a gas station cigar stuck in the intake. The ones I've been looking at have a can or a jar with baby oil, a soldering iron, and a low-pressure air feed, which AFAIK is pretty close to how the proper ones work.

 

Without a smoke machine, you can hold a length of hose to your ear and poke the other end around the running engine, and maybe home on the noise that way. You can also shoot carb cleaner at where you think the leak is and see if the idle picks up from the engine sucking in fuel, but I've never had much luck with that method.