1993 RX-7 Idle Quick Rev Engine Rough

[jamesg302]

Hello. I'm looking for some advice from the great people here at RCC. We have a 1993 RX-7 that is stock except for removing the pre-cat and installing a down pipe.

Brief history. About a year ago the car was being driven home from work at night and experienced a sudden over-heating followed immediately by massive coolant overflow on the passenger side of the engine. The car had to be trucked home as it would not run and could not be driven. Prior to starting any work error codes were checked and there were no error codes. After some investigation it appeared that the thermostat was stuck closed and was therefore replaced. We also discovered that the L2 spark plug wire was arcing immediately at the connection to the coil as the end of the wire was corroded through and basically fallen off in the coil plug. The leading coil was tested and found to be outside of specification (low resistance) and was replaced along with the spark plug wires. At the same time as this work the opportunity was taken to replace the vacuum hoses.

After putting everything back together and testing the car there is a problem. Starting the car cold and warm yields a fairly smooth idle at 700-950 rpm. However, if you quickly hit the gas and let off the rpm will drop to approx. 400-700 and the car will begin to run very rough along with a knock or pinging sound. Depressing the throttle to raise the rpm to 800 does not smooth out the engine. If the engine is turned off and back on the car will idle fairly smooth again at 700-950 rpm. When road driven the engine will attempt to quit when stopping at lights unless more gas is given by the pedal. The only error code is #38 Accelerated Warm Up. We have not troubleshooted the error but given that this solenoid is only activated to quickly warm up the car by keeping the rpm high on start it seemed unlikely the a faulty solenoid would create the problem we are experiencing. Other symptoms/observations. Vacuum seems good and is 20 to 12 on idle. Boost seems good; goes to approx. 12 on acceleration. Oil pressure varies greatly. Is this normal? Oil pressure on acceleration goes almost to top of gauge (100+-) and on idle or deceleration goes down to bottom 15%.

Any advice or suggestions for troubleshooting would be greatly appreciated. Thanks in advance.

- James

[Garfinkles Motor Works]

Remove the idle speed control and clean the thing with carb cleaner .

[Signal 2]

The low indicated oil pressure at idle...especially the low rpm idle you mentioned is typical. After you clean the idle speed control, locate and clean the single wire connection at the oil pressure sensor. It's just below the oil filter. Frequenly cleaning the connection improves the accuracy of the indicated pressure at idle, since at idle the signal is weakest. If your oil is thinned by fuel dilution or has a low viscosity rating, this will also affect what you see on the gauge...especially at idle.
In some cases you may need to change the oil pressure sensor.

[jamesg302]

Thank you both for the suggestions. I removed and cleaned the idle speed control solenoid and tested the resistance while it was off. The resistance is 11 and therefore within the 10.7 to 12.3 specification. After reinstalling it I tested the car again and found the following symptoms.

== At Cold Start ==
Idle: 1500-1700 (low probably due to accelerated warm up solenoid possibly bad, resistance tested on car at 16 outside of 9.3 to 11.3 spec.)
Oil pressure: on mark immediately below 120
Boost/Vac: 15 vac

After warming to slightly above "C" quick throttle rev to 2k and returns to 800rpm.

== At Warm Start ==
Idle: barely 500 and will quit if no gas given by accelerator
Oil pressure: 60
Boost/Vac: 12 vac

Quick rev to 2k and release causes car to backfire then drop to 100rpm and quit.

I noticed that the fast idle cam may be out of adjustment but would it cause any of the symptoms? Is it possible that any idle settings are out of adjustment given that the dealer may have adjusted them with the original vacuum hoses and some vacuum leak that has now been corrected by the installation of new vacuum hoses?

Thank you for taking time from your busy schedules to help others with your expertise and experience.

- James

[Signal 2]

Check page A-12 in the FSM. If you had a vac leak when the idle was messed with it could be at least some of your issue.
Sounds like your indicated oil pressure is fine though.

[jamesg302]

Thanks Signal 2. Any idea what the relationship is between the idle screw and rpm? Such as 1/4 turn equals 100 rpm, etc?

Seems strange that there are 2 manual adjustments for the idle (the screw and throttle cable) when I thought the idle would be fully controlled by the idle control solenoid. How can these manual settings have any affect if the solenoid also opens and closes to allow more/less air at idle?

[scotty305]

The engine needs a certain amount of air to idle at a certain speed. There are quite a few possible air paths into the engine, depending on how you look at things:

1. Main throttle stop. This can be adjusted using a set screw. I'll have to double-check but I think it's on the front of the throttle (closer to the front of the car).
2. Fast idle cam. This is on the rear (firewall side) of the throttle and uses a thermo-wax pin to increase the throttle stop (from the back side of the throttle shaft) and allow additional airflow when the engine is cold.
3. Air bypass screw. This near the bottom of the throttle, and can only be seen with the intake elbow removed.
4. Idle Speed Control solenoid, located on the rear of the upper intake manifold. The ECU can vary a certain amount of airflow by opening this solenoid valve between 0-100%.
5. Accelerated Warm-Up Solenoid (AWS), located under the upper intake manifold. The ECU can open or close this solenoid.
6. Vacuum leaks can allow air past the throttle and cause the car to idle high.

Items 1-3 could be manually adjusted to compensate for a vacuum leak. The stock ECU may not be able to reduce total airflow enough to compensate for a large vacuum leak.

[jamesg302]

I still need help!

We adjusted the air bypass screw under the elbow. The screw had to be opened 4-5 turns. The dash pot was also adjusted because it was set to engage a little low (2300rpm instead of the 2600-3000rpm). But there is still a problem with the following symptoms.

== At Cold Start ==
Idle: 1500-1700 (low probably due to accelerated warm up (AWS) solenoid, its probably bad, resistance tested on car at 16 outside of 9.3 to 11.3 spec.)

== At Warm Start ==
Idle: 750-775rpm (680-700rpm with 10pin and GRD jumped)

The problem described as tested with engine warm:
- The car idles fairly smooth; kinda like a fast putt putt putt.
- You can now rev the car to 4000rpm and it returns normal to idle.
- If the car is driven without boost (i.e. without stomping on the gas) when coming to a stop, taking the car out of gear, and idle in neutral the car will idle fine.
- If the car is driven with boost (almost full throttle in first gear gets 5-6lbs boost) when coming to a stop, taking car out of gear, and idle in neutral the car will idle very rough (like a pinging/knocking) at 550-900rpm. Seemed to have higher boost before idle screw was adjusted.
- If the car is started and nothing is touched it idles fine. If you turn on anything that draws an engine load (A/C, headlights, fan) the idle is very rough (like a pinging/knocking) and is 800-1250rpm.
- In either of the above scenarios if the car is turned off and immediately back on it will idle normal until one of the above triggers occurs to cause it to idle rough.

Any thoughts would be greatly appreciated as I'm not sure where to test next. It is curious that the problem goes away when the car is turned off and back on. That symptom seems consistent with the previous problems. Could that mean electrical problem, stuck solenoids, ECU (ugh!)?

Thanks for your help. It has been invaluable so far. I hope you guys have some great suggestions or even better a story where you experienced the exact same thing and can tell me the solution.

- James

[jamesg302]

Hello and thank you all for the suggestions. We have tested some things and still have more to test. However, I have more specific details on the problem and wanted to share them here for some suggestions.

1/21/2011

== At Cold Start ==
Idle: 1500-1700 (low probably due to accelerated warm up (AWS) solenoid, its probably bad, resistance tested on car at 16 outside of 9.3 to 11.3 spec.)

== At Warm Start ==
Idle: 750-775rpm

The car's idle both cold and warm is still rough. We tested adding different loads both cold and warm to try and narrow in on the cause of the significant drop in rpm and knocking. The following loads were tested.

HVAC Fan (speeds 1 & 3), AC (On/Off with fan speeds 1 & 3), Headlights with high beams (On/Off), Radio (On/Off), P/S (load), Clutch (In/Out), Read window defroster (On/Off), Brake pedal (depressed)

Of the above loads mentioned the computer raised the rpm to accommodate all loads. However, when the AC was turned on the engine immediately went from the rough idle to very rough engine shake with knocking. Turning the AC on while depressing the Brake pedal would cause the rpm to drop around/below 500rpm and the knocking. The knocking persists when the AC load is removed but disappears if the car is turned off and back on. Combining other loads to create a heavier electric load (i.e. radio, high beams, and defroster) had no effect.

I mentioned before that using the turbos seemed to have an effect on the knocking and rough engine. We retested and found that turbo usage has no noticeable effect on the knocking or idle. The problem seems to be linked to engine load at idle (800-1500 depending on load).

Is it possible that this symptom is being caused by a lean fuel condition? Many sites mention that a certain amount of modifications require the computer to be re-programmed. This car's only modifications are a down pipe (eliminated pre-cat) and an after-market exhaust single outlet muffler (installed by previous owner).

Also, the clutch switch, stoplight switch, and knock sensor were tested and are OK.

Please let me know your thoughts. Thanks.

Regards,
James

[jamesg302]

Hello again. We have completed some more testing and would like your feedback/suggestions.

2/19/2011

The MAP sensor was checked and is within specification.

A compression test was done with the car run hard then tested right after 10 minute cool-down. The circuit opening relay was removed, the coil box under the cruise control was disconnected, and both trailing spark plugs were removed. We used a standard compression test gauge with the valve removed. The followed average readings were taken at the cranking RPM for each side of the rotor.

== Back Chamber ==
80 PSI
78 PSI
80 PSI

== Front Chamber ==
65 PSI
45 PSI
30 PSI

The car seems to drive fine except for the rough idle which is now evident even without the extra load of the A/C turned on. The 3rd gear boost test at WOT yields 10-8-10.

Is it possible that one of the front chamber apex seals is stuck or do the PSI numbers indicate that the seal must be cracked, chipped, missing?

We added SeaFoam to the gas and a small amount to the oil in an attempt to un-stick the seal if in fact that is the problem and put one gallon of water through the intake to clean any carbon deposits that may have existed.

What is the risk of running the car in this condition if a seal is bad? The car is manageable to drive and the $4,500 engine re-build cost is difficult to accept at this time.

Thank you again for taking the time to read this post and offer your advice.

Regards,
James