derSchwamm

iTrader: (1)

I am still new to rx7s so bear with me here!

I bought this car two months ago and it constantly idled between 1500-2200 rpm regardless of temp, unless I tapped the TPS adjust screw which made it drop to 800. After lots of tinkering I figured the throttle shafts were worn and got a different throttle body from someone on the forum here. I installed it, reset the idle, and now have more or less a smooth 800 rpm idle.

Problem is, it idles slow even when cold. When I start the car, it drops immediately to 800rpm rather than holding at 1500 and dropping as the engine warms up. It's difficult to manually hold the idle higher, and I don't want to damage my engine. When I installed the new TB I noticed on the new one the two flaps on top (air filter side) were closed, whereas they were open on my old throttle body. I guess maybe the thermowax isn't functioning? I can upload a pic of the TBs side by side if it helps. I already swapped over the vacuum house from the old thermowax but it didn't help. The diaphragm seems to be intact: it doesn't move when I plug the vacuum hose with my finger but it will move back to open when I let go. What else can I test? I am not sure exactly how this cold idle system is supposed to work.

The second issue I have is that the idle will hang around 1100 rpm until something else places load on the engine, then it drops to 800. There is a description of the problem in the gsl-se idle troubleshooting guide at nellump.net, but following their "fix" doesn't work. Any advice would be appreciated. Maybe one day this car will idle like it should!

Cameron38

iTrader: (2)

Are you setting the fast idle by pumping the gas pedal once before starting? Don't feel bad, it took my mechanic (who has been working on RX's for 30+ years) several tries to get my SE to idle correctly cold or hot. Problems can relate to Throttle body, Vacuum hoses, Wax Pellet, TPS and more. My idle will still sometimes hang around 1100 rpm and then just as mysteriously return to normal. All part of owning a car with a first generation FI system.

derSchwamm

iTrader: (1)

I did actually know about the fast idle cam and depress the throttle all the way before starting, but I can't tell any difference.

Here's a picture of my two throttle bodies. The original one, which idled very high, is on bottom. The replacement, which will correctly idle at 800rpm, just not at 1500 when cold, is on top. You can clearly see the flaps are in different positions even though both are cold (and out of the car, obviously). I'm assuming one is wrong (top one?) and I need to track down the cause.

LongDuck

When cold, they should be closed - as this acts like a choke to ensure a rich mixture during cold startup. Therefore, the one on the bottom has the choke throttle rods in the incorrect position for a cold thermowax pellet actuator. Behind the choke plates are the secondary throttle plates - which are opened as needed during acceleration. As you can see, if the choke plates are always closed, the secondaries will never get air to them to be throttled, hence getting this system to work correctly is critical for proper engine performance when warmed up.

For clarity sake, the primary bore is on the top in your pictures, with the secondaries on the bottom. The thermowax pellet is heated by engine coolant circulating through it's housing which eventually melts the wax, allowing the choke rod to rotate the butterfly valves into a 'normal' position, bypassing air.

In your case, the thermowax pellet is likely broken internally (broken spring?) which is preventing the choke from working correctly when cold. Also, recognize that the cold start cam is a mechanical linkage on the right side of the throttle body which is 'set' to the fast-idle position when you push the gas pedal all the way to the floor and then release it before you start cranking. When cold, this sets the idle speed higher than normal to warm up cats for best efficiency and to help warm up the engine coolant to run interior heat for cold climates. When hot, the fast-idle cam is not actuated to 'set', per the thermowax pellet.

Quick fix would be to swap the thermowax pellet assembly from your original to the replacement throttle body and see how that works. Fast idle is really only important for the stated reasons above, as any other condition of fast idle on startup is to get solid oil pressure going on cold days. Where I live (the desert) engine heat isn't that important, nor is coolant heat to run the interior heater...

derSchwamm

iTrader: (1)

Thank you, that's very helpful. I now have a better idea how this all works.

I was curious though... I understand the rationale of swapping thermowax pellets to see if it fixes my problem, but if the bottom (original) throttle body here is the one that is incorrect, wouldn't that mean the original thermowax pellet is broken? Or are there other things that could cause that throttle body to be in the incorrect position?

I was standing in several inches of snow while working on my car today, so no, I do not live in a desert, and I've been concerned about starting it in this condition with these outdoor temperatures (0-25º ish). You mention the purpose of this higher idle is to get the heater working faster, so does that mean I won't be causing any harm to my car if I start it as-is for a little while? I've been letting it sit while I figure this out, just to be safe.

LongDuck

There's no problem with driving the car cold on a low idle. It just won't heat up very quickly and hence you won't have any coolant heat to run through your heater core to warm up the interior.

After reading your response, you might actually have 2 separate things going on here that are separate but seem to be related;

1) Fast Idle Cam - This is a mechanical linkage that serves as a stop for the throttle return spring and the primary throttle rods to rest on when the engine is cold. Instead of returning to the idle adjust screw (tiny brass flat-head screw mounted to the throttle body plate) when the engine is warm, the fast idle cam holds the throttle higher to keep engine RPM up - which results in more gas being burned, more heat being generated, and more heat then getting to the coolant to open the thermostat, flow through the radiator and then get to your heater core for interior heat. There's not much additional benefit to a fast idle when cold other than better oil pressure as mentioned above - but I was always taught NOT to romp on an engine when it's cold because the oil isn't flowing properly until it's warmed up. A buddy's BMW Z4-M even has a throttle LIMITER when cold and lights on the tachometer to let him know how far to hold back until the engine and oil warms up sufficiently to ensure proper lubrication.

2) Choke, aka Thermowax Pellet - This is the heat-based (coolant driven) mechanism which holds the secondary throttle plates (choke plates) closed. When closed, the air is restricted allowing for a rich mixture during all driving conditions until the engine warms up, the coolant gets hot and flows to the Thermowax Pellet, melting the wax and allowing the internal springs in the Thermowax Pellet assembly to expand, thereby opening the choke plates and flowing air past to the secondaries. If this system isn't working as in your picture above - i.e., the choke plates are horizontal to the airflow - you get no choke effect because the air is free to flow past the choke plates. This isn't a serious problem unless the engine runs so lean that it won't drive smoothly when cold. Part of having a high idle on cold start is that the engine speed is increased to help slightly when shifting, but this can be mastered with heel/toe technique and driving style.

Mazda designed these cars before the technological day and age when the ECU could actuate all of these linkages and vacuum controls to make this work correctly. Our cars ECU takes in limited information and sensor readings and routes these out to indicator lights and makes minor changes to fuel injector pulse timing and duration during closed-loop operation (5th gear switch, primarily), otherwise, it's just feeding injector pulse information in 12v to the injectors according to a set map which never changes, and can't be programmed or altered in any way. It's pretty basic as far as ECU's go, and I don't think it even stores information about Check Engine Codes (flashes in a sequence) - only displaying the codes for sensors that are currently out of spec.

Anyway - back to the issue at hand; I would feel comfortable driving the car without a fast idle set on startup, but you may find that driveability is affected until the car warms up. The alternative is pretty annoying (search here on 'SE surging') and if you don't have that - you're already ahead of the SE tuning game! Good luck,

derSchwamm

iTrader: (1)

Alright, I've done more work and wanted to provide an update. I think I've fixed everything.

I took the throttle body that constantly idled at 800 rpm apart and cleaned everything, then swapped all the external pieces, from the individual pieces on the shafts to the thermowax, vacuum hoses, and fast idle cam parts from my original throttle body to the replacement. It looked like a brand new throttle body by the time I was done with it. When I started the car it revved right up to 1500 and dropped to about 800 within a few minutes. After adjusting the TPS and all the car seems to run perfectly. It drops to about 1100 rpm when I come to a stop for just a few seconds, then drops to 800. I guess that's normal. I'll report back if I continue to have any problems. Thanks for the help!

LongDuck

When slowing down to come to a light from higher revs, if the RPM stays at 1100 and then gradually drops back down to normal idle - that's a slow dashpot.

The dashpot is designed as a static vacuum source with a spring inside and when the throttle comes down, a protrusion of metal hits the dashpot plunger to prevent the throttle from slamming shut suddenly. Yours is working TOO hard. If your other TB didn't do this, grab the dashpot from that one and put it on this one.

As you come to a stop, the dashpot operation to prevent sudden throttle closure should be entirely seamless to the driver, as it all happens pretty quickly when working correctly. HTH, and glad you got it fixed!

LongDuck

By the way, if you have not already read this - Felix's is a GREAT place for general rotary knowledge and specific to -SE's. Have a look here for some great info: Felix's How-To/How-It-Works Garage

Take care,

derSchwamm

iTrader: (1)

I am not actually sure it is the dashpot. I have had two different ones on the car and it behaves the same way. To be clear, the car drops to idle like normal, except it gets hung up at 1100 for a few seconds, before dropping again straight to 800. Sometimes it's just a little blip, other times it hangs for 10+ seconds.

I have read the the vent solenoid valve and vacuum solenoid valve can have something to do with this, but I do not have spares that I could swap on. I also see there is another device called the vacuum control solenoid valve, which the GSL-SE idle guide says this about: "It's job is to return the spark timing to its most retarded setting during deceleration and idle." Sounds like another potential culprit?

Do you know how I might go about testing these? At this point the 1100 rpm hang-up is just an annoyance, but I'd like to get it fixed.

LongDuck

FSM tests for VSV function (and the other vacuum solenoids) have to do with either allowing a vacuum to be pulled through it from the intake under 12v - creating suction, or allowing the intake air to vent under 12v, creating pressure. Each valve receives it's source air from the 'spider' under the upper intake manifold. The 12v is switched by the ECU.

You could check function on these and find replacements fairly easily.