Rough Idle/Dies - I know, vaccum leak but...
 

I swapped an '87 engine into my '86 and now the car is idling rough (when it idles) but just dies most of the time after start up (if I'm not on the gas).

The engine still runs rough under 2000 RPMs when I'm on the gas, but past that it's extremely smooth feeling. I understand that it is possibly a vacuum leak; however, I can't find any vacuum line that isn't hooked up.

I'm thinking maybe a line is hooked up incorrectly so I'm trying to find close-up pictures of the vacuum lines in place. I've searched the FAQ and past topics and haven't found any pictures.

Anyone here have some or could give me advice on the problem?

Thanks.

did you hook the large rubber hose up on the under side of the intake tube, the large tube that goes from air box to TB, that hose runs over to the BAC wich will kill the car if you have the hose removed

also you can keep it running and take a water hose and spray water in different areas to trace the vac leak if it isn't that hose, once the water kills the engine you are in the right place...

Yes, that hose is connected.

I'll be sure to try spraying water on the hoses and get back to you.

Thanks again.

Ok, tried that method. Now there's some loud hissing sound and I'm assuming it's my leak. It started after we sprayed the yellow plastic coupling that runs two hoses down (one down to the upper intake and one to a short green cylinder that continues to the other side of the throttle body). These hoses we checked and are fine, no cracks or tears. So then we put them back on and still the same problem.

Now we've taken the upper intake off again to check the vacuum hose assembly. Everything appears to be fine.

Going to put the upper intake back on and see from there.

A vac leak can also come from worn out, aging, or reused intake gaskets. It doesnt just have to be a vac line.

A story I know all too well. I gave up on my problem engine, never tracked it down.

https://www.rx7club.com/2nd-generation-specific-1986-1992-17/find-vacuum-leak-win-prize-721618/
https://www.rx7club.com/2nd-generation-specific-1986-1992-17/still-not-idling-radiator-smoke-sad-panda-740056/

Check the fuel and omp injectors. Check all the gaskets you can find. Pray for luck.

Don't forget that the vacuum leak need not be in the hose itself, it may also be in the device the hose goes to. I am not sure which line you are describing, but it sounds like it might be the 'double throttle diapragm'. The line goes from the UIM to a thermovalve on the back side of the throttle body, and then another line goes from there, around the right side of the throttle body under the intake tube, through a little plastic delay valve, and then continues to the front side of the throttle body where it connects to the double throttle diaphram. I *think* the 'yellow plastic coupling' you are describing may be the thermovalve.

The leak can be inside the throttle diaphram (ask me how I know this), in the delay valve, or even the thermovalve, as well as in any of the three hoses or their connections.

If the thermovalve is operating correctly, with no leaks up to that point, the problem should go away when the car warms up and the thermovalve closes.

If the thermovalve is staying open, the leak could be anywhere in that system. A quick way to check would be to pull off the line that goes to the thermovalve from the UIM and cap it off at the UIM. If the problem stops, the leak is in that system. If not, it is somewhere else.

Also: if you put an 87 engine into an 86 car, there is the potential for a mismatch in the impedance of the injectors. They changed to a higher impedance midway through the 87 year. If you have high impedance injectors in a car that still has the resistor in series, the injectors may not be getting enough current to operate correctly. Easy to check with an ohmmeter; specs are in the FSM.

Check the O2 sensor voltage at idle. It should be rich (greater than 0.45 volts.) If it is not, the car is running lean, especially if this is so after it is warmed up. It should show rich all the time during idle. If it is lean, that could be a symptom of a vacuum leak, or clogged injectors, low fuel pressure, bad fuel regulator, etc.

Is the ECU showing any trouble codes?

Thank you for the responses.

After putting the upper intake/TB back on, the car was able to start up and it was smooth through all RPMs. Unfortunately, the car still died while it was cold. After it warms up, it idles at around 750-800 RPMs and that's with the idler screw adjusted all the way up!

Here's what I have noticed. Before the engine was taken out of my '86 (note: it was another engine from an '87) there was a hissing sound coming from somewhere. I was never able to find it and I hoped it would get fixed in the engine swap. The car still has a small hissing sound, so my assumption is it isn't from the engine itself.

I'll try the things stated above, but would anyone happen to know where my leak could be if it's not on the engine itself? It's not a very loud sounding leak, but it's noticeable.

More than 100 places.

The generally accepted way of finding a vacuum leak is to go over all of the lines and the devices they connect to with a spray or gaseous fuel, such as starting fluid, carb cleaner, or butane. All of these involve fire hazard. When you put the fuel over the leak, the rpms should increase.

Be *very* careful with any of these processes. Wear eye protection, and work in a well ventilated area, and away from the water heater.

Trace every vac line from its origin on the UIM, to its ultimate destination at the device it operates. Check also around the operating device, and any of the vacuum solenoids (4 of them on the left side of the engine).

Where are you in Wisconsin? If you're close I may be able to lend a hand, i just spent the past afternoon fixing leaks/idle issues on my '87 that i brought back to life after sitting for about 6 years. Do you still have a BAC valve on this engine you swapped in? Does the car die when you unplug it?