Mechanics

MAZDA RX7 RPM PROBLEM

1982 Mazda RX7 • 130,000 miles

My 1982 Mazda RX7 GSL drops down and idles very slow (
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Dexter Dyer
November 22, 2012.



Is this a carburetted or injected model?
Have you tried adjusting the idling speed?


KHLow2008
Nov 22, 2012.
It is a 12A engine with the standard Mazda carburator (no changes have been made as I am the original owner and have kept it pure stock).

When the idle operates properly (i.E at 750 rpm) I can adjust the idle speed just like it says in the manual and it is very sensitive even to a 1/4 turn of the idle screw but when it is in this low idle mode (


Tiny
Dexter Dyer
Nov 22, 2012.
This seems a little tricky. Check for vacuum leaks. This can result in erratic idling. Were spark plugs and wires checked?


KHLow2008
Nov 22, 2012.
Both myself and the mechanic that I have been working with have checked for vacuum leaks so if I have one, it is subtle. The spark plugs were changed recently as it has always run rich and tends to need them changed about 2X per driving season. The wires are about 5 years old so I plan to change them (as well as the distributor and cap) in the spring since the car is now off the road for the winter. I can still do any static tests that don't require driving as the car is on blocks.

One more bit of anecdotal information. When the car idles poorly, if I check the TPS test signals, there is about a 1 second gap between the 2 signals coming on when you decelerate from 3000 rpms. The shop manual says that they should come on at the same time but we can't adjust them to that even with the new TPS sensor. However, when the car idles properly, there is a 3-4 second gap between the 2 signals and again the adjustment procedure has no effect on the signal timing. In addition, when the car idles poorly, the rpms drop slowly to about 1000 rpms and then it is like it drops off a cliff to


Tiny
Dexter Dyer
Nov 23, 2012.
When decelerating, only one signal should be present. Two other components are related to decelerating from 3000 rpm, anti-after burn valve and dashpot. Were they checked/tested and also the by-pass air control valve?


KHLow2008
Nov 23, 2012.
Maybe I was not clear on the TPS signal measurement. Do you have the shop manual? If yes, in section 1A: 13 there is a TPS measurement section 1A-D-4. I followed the steps and disconnected the coupler (BY, GB). Then I measured the signals at test points (GY, LgY). That is where I observed the timings that I mentioned above. Do you have other recommended places to measure?

I just completed the tests for the anti-after burn valves #1 & #2 - they checked out OK. I assume that the dashpot is part of the valves - correct?

I am not sure what you mean by the by-pass air control valve so if you can reference it to the shop manual or describe what it is and where it is located I can test it.

Lastly, on another web thread, they recommended checking the vacuum advance on the distributor. I completed that static vacuum test and both worked properly and held vacuum pressure.

I appreciate your help and as it is getting cold here in the northeast, the time I can spend in my barn working on it is limited but I will keep trying as long as you have suggestions.


Tiny
Dexter Dyer
Nov 28, 2012.
I only have the 1984 model shop manual with TPS sensor adjustment. The 1983 shop manual does not have any information for adjustments.

My previous comment is not correct as I was refering to fuel injected models. Here are the procedure from our database.

1. Warm engine to normal operating temperature. Stop engine and connect tachometer. Disconnect Brown connector under air cleaner on carbon canister
side of engine.

2. Using 2 voltmeters or 12-volt, 3-watt bulbs, connect negative lead of each voltmeter to each terminal in connector. Connect positive leads to "B" terminal of alternator. See Fig. 10.

3. Start engine. Quickly decelerate engine from 3000 RPM and make sure that current flows to both voltmeters or bulbs at the same time. If current does not flow at the same time, adjust throttle sensor.

4. To adjust sensor, remove cap from adjusting screw. Adjust timing of current flowing to voltmeter "A" in Fig. 10 by turning adjusting screw. Turning screw clockwise causes current to flow earlier and turning screw counterclockwise causes current to flow later. See Fig. 11.

5. After adjustment, install cap over throttle sensor adjusting screw. Remove voltmeters and reconnect Brown connector. Remove tachometer.

Dashpot is only applicable for manual transmission, is your vehicle manual or automatic?

I am no specailist on this vehicle so my help is limited.


KHLow2008
Nov 29, 2012.
The TPS adjustment procedure is very similar to the one in my '82 manual. As I stated previously, the weird part is that when we do the adjustment procedure, we can only the the currenst to come on about 1 second apart even with the brand new sensor and that's when it idles poorly. When it idles smoothly at 750 rpm, that time gap is much larger and we cannot change it either. It is strange.

Yes, I have a manual transmission and I'll take your diagrams and see if I can find the dashpot. Believe me, the diagrams you sent are better than the crappy pictures in the 1982 manual. Is there a 1984 manual online that I could download? One of my irritations, is that there is no theory of operation in my manual so I could see if some of the timing sequences aren't right and as I said, it has poor pictures instead of diagrams.

One other thing, I ordered a new hose that goes from the back of the manifold to the anti-afterburn valve. Mazdatrix in California has them and says that they tend to crack and loose some vacuum. It should be here next week.

No apologies, if there are any experts left on these cars, they are probably 90 years old and in Japan. Keep suggesting things for me to try and if I solve it, you'll bethe first to know.


Tiny
Dexter Dyer
Nov 29, 2012.
The information I get is online, from Mitchell1.

http://www. .com

Check the throttle shaft for excessive side movement. This would cause erratic idling and also can result in the inability to adjust the sensor. Test throttle at various throttle opening.


KHLow2008
Nov 30, 2012.