88 GXL won't hold a steady Rev
Thread Starter
Junior Member
Joined: Mar 2010
Posts: 14
Likes: 0
From: Okotoks Alberta Canada
88 GXL won't hold a steady Rev
Hello,
Alright, I've recently acquired a 88 GXL RX-7, and the car does not hold any RPM's between idle and 4,000 RPM before it gets up to operating temperature. Once it get's up to operating temperature, it's fine, and the car idles fine to. But if you rev it up when it's still cold, the car will rev up and then drop down and then rev up and then drop down in a noticeable pattern.
I am a decent Mechanic (although still an apprentice) so don't worry about being technical, unless it pertains only to the rotary engine. But I know so little about rotary engine's that this has me stumped. I am guessing it is a TPS sensor though.
Any help would be appreciated.
Alright, I've recently acquired a 88 GXL RX-7, and the car does not hold any RPM's between idle and 4,000 RPM before it gets up to operating temperature. Once it get's up to operating temperature, it's fine, and the car idles fine to. But if you rev it up when it's still cold, the car will rev up and then drop down and then rev up and then drop down in a noticeable pattern.
I am a decent Mechanic (although still an apprentice) so don't worry about being technical, unless it pertains only to the rotary engine. But I know so little about rotary engine's that this has me stumped. I am guessing it is a TPS sensor though.
Any help would be appreciated.
Senior Member
Joined: Aug 2009
Posts: 479
Likes: 0
From: Los Angeles
Could be any of several things. Here's some ideas:
The occurence at cold temps points towards a possible problem with the accelerated warm up system. It senses coolant temp via the water thermo sensor. It has a two pin connector just behind and below the alternator. On a cold start, the ECU senses a high resistance on that sensor, and commands the air bypass solenoid to open; air then flows through its hose into the dynamic chamber, bypassing the throttle. the idle should be 2500-3000 rpm for 17 sec after start if it is cold enough to trigger the system.
I would look for an intermittent themo sensor, or a problem with the air bypass relay (left side of the dynamic chamber, below and behind the BAC valve). Try pinching off the hose leading to the solenoid with a pair of pliers while the oscillation is occuring. If it stops, you are on to the culprit; if not, it is somewhere else.
Another place to look is the double throttle system. The diaphram that actuates the double throttle gets old and develops holes. At low temp, the temperature-sensitive valve on the back of the throttle body allows vacuum to reach the diaphram, through a delay valve. At operating temp, it closes off. If the diaphram is holed, it might be admitting extra air when the temp is cold, and not when the engine is warm. You can check this theory by clamping the vac line to the valve during the fault; if it stops, you found it.
Also, there are two intake air temperature sensors, one in the air meter and another on the left side of the dynamic chamber. Either one could be going intermittent in a particular temp range.
The occurence at cold temps points towards a possible problem with the accelerated warm up system. It senses coolant temp via the water thermo sensor. It has a two pin connector just behind and below the alternator. On a cold start, the ECU senses a high resistance on that sensor, and commands the air bypass solenoid to open; air then flows through its hose into the dynamic chamber, bypassing the throttle. the idle should be 2500-3000 rpm for 17 sec after start if it is cold enough to trigger the system.
I would look for an intermittent themo sensor, or a problem with the air bypass relay (left side of the dynamic chamber, below and behind the BAC valve). Try pinching off the hose leading to the solenoid with a pair of pliers while the oscillation is occuring. If it stops, you are on to the culprit; if not, it is somewhere else.
Another place to look is the double throttle system. The diaphram that actuates the double throttle gets old and develops holes. At low temp, the temperature-sensitive valve on the back of the throttle body allows vacuum to reach the diaphram, through a delay valve. At operating temp, it closes off. If the diaphram is holed, it might be admitting extra air when the temp is cold, and not when the engine is warm. You can check this theory by clamping the vac line to the valve during the fault; if it stops, you found it.
Also, there are two intake air temperature sensors, one in the air meter and another on the left side of the dynamic chamber. Either one could be going intermittent in a particular temp range.
Thread Starter
Junior Member
Joined: Mar 2010
Posts: 14
Likes: 0
From: Okotoks Alberta Canada
Could be any of several things. Here's some ideas:
The occurence at cold temps points towards a possible problem with the accelerated warm up system. It senses coolant temp via the water thermo sensor. It has a two pin connector just behind and below the alternator. On a cold start, the ECU senses a high resistance on that sensor, and commands the air bypass solenoid to open; air then flows through its hose into the dynamic chamber, bypassing the throttle. the idle should be 2500-3000 rpm for 17 sec after start if it is cold enough to trigger the system.
I would look for an intermittent themo sensor, or a problem with the air bypass relay (left side of the dynamic chamber, below and behind the BAC valve). Try pinching off the hose leading to the solenoid with a pair of pliers while the oscillation is occuring. If it stops, you are on to the culprit; if not, it is somewhere else.
Another place to look is the double throttle system. The diaphram that actuates the double throttle gets old and develops holes. At low temp, the temperature-sensitive valve on the back of the throttle body allows vacuum to reach the diaphram, through a delay valve. At operating temp, it closes off. If the diaphram is holed, it might be admitting extra air when the temp is cold, and not when the engine is warm. You can check this theory by clamping the vac line to the valve during the fault; if it stops, you found it.
Also, there are two intake air temperature sensors, one in the air meter and another on the left side of the dynamic chamber. Either one could be going intermittent in a particular temp range.
The occurence at cold temps points towards a possible problem with the accelerated warm up system. It senses coolant temp via the water thermo sensor. It has a two pin connector just behind and below the alternator. On a cold start, the ECU senses a high resistance on that sensor, and commands the air bypass solenoid to open; air then flows through its hose into the dynamic chamber, bypassing the throttle. the idle should be 2500-3000 rpm for 17 sec after start if it is cold enough to trigger the system.
I would look for an intermittent themo sensor, or a problem with the air bypass relay (left side of the dynamic chamber, below and behind the BAC valve). Try pinching off the hose leading to the solenoid with a pair of pliers while the oscillation is occuring. If it stops, you are on to the culprit; if not, it is somewhere else.
Another place to look is the double throttle system. The diaphram that actuates the double throttle gets old and develops holes. At low temp, the temperature-sensitive valve on the back of the throttle body allows vacuum to reach the diaphram, through a delay valve. At operating temp, it closes off. If the diaphram is holed, it might be admitting extra air when the temp is cold, and not when the engine is warm. You can check this theory by clamping the vac line to the valve during the fault; if it stops, you found it.
Also, there are two intake air temperature sensors, one in the air meter and another on the left side of the dynamic chamber. Either one could be going intermittent in a particular temp range.
And I would have never of thought about the double throttle system. Mostly becase I didn't even know that it HAD a double throttle system! lol
But I will check it out later on today, should have the flywhel back from being resurfaced today, and then I can slap the clutch back on there and the exhast and do some diagnostic.
Thanks again! Very helpfull post!
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