Low idle when hot

[mattmoox]

My 92 SVX with a manual transmission has developed a low idle issue.

Here’s a description of the issue:
Once the engine is fully warmed up, when coming off the gas the idle drops to ~600 rpm, then gradually drops to ~200 rpm over the course of a few seconds. While it sits at ~200 rpm, it is a little lumpy. Happens all the time when the engine is hot; does not seem to be related to any environmental conditions. When the AC is turned on, the idle raises up to ~700 rpm, but then slowing drops down to ~200/300 rpm.

After researching lots of threads on this forum, I’ve tried the following things, with no improvement:
- Replaced TPS, adjusted it 0.5V, reset ECU
- Swapped MAF with another (both read ~1.2V on the white signal wire at idle, which I think is about right).
- Replaced fuel filter.
- Checked Coolant Temp Sensor when warm - it seems to be in spec at ~200 ohms when the engine is hot.
- Removed and cleaned PCV valve. Didn’t replace it, as it seemed to be moving freely, and would only pass air in one direction.
- Disconnected left, right, and both O2 sensors (now back reconnected).
- Cleaned and adjusted IAC to various (now back to stock position).
- Removed air box leaving air intake open to engine bay air (now back to normal)

Thinks of note with my car:
- 1992 with manual transmission
- 120k
- California car
- Neutral Safety Switch is permanently grounded
- Running ECUTune Stage 1v5 ROM
- Removed noise suppressor from ECU a.k.a “resistor crush fix”

Looking for other ideas for what might be causing this.
- A/F mixture problem at idle?
- Leak in EGR?
- Fuel pressure problem?
- Leaky injector?

Thanks for any ideas to help to track this down.

Matt

[kwren]

does the Check engine light come on??

Keith

[mattmoox]

The CEL does not come up, and there are no codes.

[kwren]

It happens only when the ac is on?

Keith

[mattmoox]

Just to clarify: issue happens with AC on or off.

[kwren]

If it will continue to run even though there is lo-idling, I would just keep it running with the rough idle a while and see if it picks up a code. Some circumstances require a bit of time to get a code-(check engine lite for the code)

Of course, keep track of the temperature

Keith

[spleen_merchant]

Hi Matt,

Your problem matches my current problem to a T. I'll get a left hand oxygen sensor code pop up occasionally though. I'm leaning towards sensor related as it behaves itself when cold. I'll be replacing the thermostat, coolant temp sensor and oxygen sensors this week to see if that fixes it.
Might be worth cross checking with my thread.

[mattmoox]

It's been happening for a few months, and has not thrown a code throughout a wide variety of driving conditions.

[Huskymaniac]

Originally Posted by spleen_merchant

Hi Matt,

Your problem matches my current problem to a T. I'll get a left hand oxygen sensor code pop up occasionally though. I'm leaning towards sensor related as it behaves itself when cold. I'll be replacing the thermostat, coolant temp sensor and oxygen sensors this week to see if that fixes it.
Might be worth cross checking with my thread.

Did replacing the coolant temp sensors work out for you?