tpursell

Hello - I'm a newbie here, so please be kind. I do not have any experience with rotary engines.

I have a 1991 Mazda RX-7 convertible, non-turbo. I picked it up as a project car and have done a lot of work. I have fixed nearly everything, but have one remaining problem that is driving me crazy. The car starts up and runs great for about 10 minutes, then suddenly decides to stall for no apparent reason. There is no use in trying to start it again for 10 or 20 minutes. It will eventually start with a struggle, but the engine has absolutely no power and knocks as though it is severely out of time. Ultimately, I have to let it sit for an hour or two before it will run again. Then it will run great for another 10 minutes.

Here are some things I have already tried/fixed:

- Replaced brake booster (the old one had a vacuum leak that stalled the engine every time the brakes were pressed)
- Replaced plugs and wires
- Replaced battery
- Replaced fuel pump (the old one seized)
- Adjusted TPS (using resistance measurements)
- Tried adjusting timing, but can't get idle low enough to even see the mark; idles about 1200-1400 all the time.

I've seen some similar problems elsewhere on the forum, but nothing exactly like the symptoms I am experiencing. Has anyone else had a problem like this? If so, what did you ultimately determine the root cause to be? What is the most likely cause of the engine failing every time it gets hot?

Thanks in advance for any insights or suggestions!

RotaryRocket88

iTrader: (7)

So it's idling around 1200-1400 RPM when it stalls? The water temp sensor on the back of the water pump housing affects the fuel mixture, so you might test it per the FSM. You can also pull engine codes: http://www.johnr.com/cpucodes.html.

tpursell

It continues to idle around 1200 or so, then when it gets hot it suddenly drops to 600 or so for a few seconds, then chokes and dies. And then it won't start again for quite some time.

I will certainly check on the temp sensor. Would it be appropriate to simply disconnect it for troubleshooting purposes? Is there a way of testing it?

Thanks! I will report what I find....

RotaryRocket88

iTrader: (7)

You can try adjusting the idle speed up. The air adjust screw on top of the throttle body will allow you to change it to a certain extent. There is also a throttle stop that can be adjusted, but unless someone has touched it in the past, it should be correct.

The hard starting afterward is probably due to it flooding, so that's going to be a separate issue. Hot-start problems are common, and there are some work-arounds. This is one of them: https://www.rx7club.com/2nd-generation-specific-1986-1992-17/looking-car-has-hot-start-problems-499744/.

As far as testing the water temp sensor, the FSM is available online here: http://www.rotaryheads.com/PDF/2nd_gen/index.html, as well as other places. There are detailed tests for just about anything. Check the fuel and emissions section.

Also, the timing is advanced by the ECU above about 1100 RPM, so it's not possible to check it if the engine is not idling properly. If you want to be sure the timing is correct, you can remove the CAS (crank angle sensor), line up the lead mark on the main pulley with the pin on the front cover, line up the mark under the CAS cover & re-stab it. I think this is covered in a Haynes manual, and there are also detailed threads in the 2nd gen tech section.

tpursell

Thanks for the additional information. Lots of things to try. May take me a few days to check it all out (I work late during the week), but I'll certainly post my findings for everyone's benefit.

tpursell

I may have fixed the problem, but time will tell...

I pulled the crank angle sensor and the gear was about 180 degrees out of sync (how did the engine run at all??). I aligned the marks and put it back in. I also took another look at the throttle position sensor and had to adjust it as far as it would go to have the correct resistance when the throttle is in idle position. I was able to get the engine started, warmed it up, and was surprisingly able to get the idle down to about 800 and set the timing as close as I could get it.

I just let the car idle for about 15 minutes, then drove it for another 15 minutes or so with no problems. That is the longest it has ran since I started working on it.

I'm going to claim success for now, but will certainly post an update if I learn anything new. Thanks to all for your ideas. There are certainly some smart people on this forum...

tpursell

Hmm. Now I'm having troubles getting it to idle properly when cold. That is not NEAR the problem as having it die when it is hot, though. Looks like I'll be fiddling with it some more...

RotaryRocket88

iTrader: (7)

It is amazing that it ran if the CAS was off by that much. The advanced timing the ECU runs while the idle speed is high must have been the only thing keeping it running. Once the RPMs drop below ~1100, the ECU switches to fixed timing of -5* & -20*, which would explain why it just killed it every time it got warm.

Do you know if the car still has the BAC valve or AWS solenoid on it? The BAC has coolant lines running through it, and is connected to the driver-side of the intake plenum. The AWS solenoid is a gold colored device connected to the BAC's air source and to the plenum as well. If one or both of these are in place and connected, cold idle speed should jump to ~3000 RPM on startup and continue for about 17 seconds. At that point, it should drop to 1500 RPM as progress down as the thermowax heats up and relaxes. From your idle description, it sounds like the thermowax is working, but the rest of the cold start system may not. You'll probably just need to give it a little throttle on cold starts until it warms a bit.

Aaron Cake

iTrader: (1)

Yeah, um, a CAS being 180 degrees out won't make a difference. The wheels are symmetrical.

RotaryRocket88

iTrader: (7)

Hah, good point. I suppose there is no designation for which side of the gear should face which direction. Maybe the "about 180 degrees" is the key here. If it wasn't completely flip flopped, the timing would still be off. There was apparently an improvement in driveability, so something changed.

tpursell

Well, I'm a little disappointed today. Haven't had the car out of the garage for a few days. It seems to be back to its old ways. Ran it about 10 minutes, and it stalled out on me again. Then it wouldn't start (seemed flooded). Guess I'll have to figure out how to get that water thermosensor out so I can test it...