14 replies to this topic

[Bill_LCFL]

I have searched and tried to find some topics, but haven't found the same one. Some were similar. I recently purchased a 91 with plans on turning it into a track day car. The engine has a weird issue. If you drive it for about an hour - within 2-3 minutes of an hour - it will begin a weird hesitation. I have figured out that at a steady pace, it will feel like a miss. If you go up hill, it feels like a power loss. If you are starting from a stop, every time you shift, the car just idles for about 3-5 seconds, then takes off.
 
The previous owner put in a stage one Exedy clutch before I purchased the car. Since then, trying to solve this I have replaced the plugs and wires, twice. I have replaced the coil pack and the fuel pump. If anyone has experienced the "left turn cut out" Miatas are known for, it feels like that. A couple of places to look, to me, would be the AFM, CAS connector and maybe TPS? What is a good way to clean the CAS connector to see if that is it? This is a video of it happening. When I shift, there is a pause before it goes again.

[Dave D.]

Have you tried another AFM? If you don't have one maybe borrow one from a friend just to test? The contacts to trigger the fuel pump relay are in the AFM,maybe you are momentarily loosing fuel pressure due to the pump not running? Can you run a multimeter from the fuel pump power terminal behind you at the tank and drive it to see if power indeed is being disrupted?

[Bill_LCFL]

I have been trying to get my hands on an AFM. The part that is just weird to me is that it has to be run for an hour or so for it to start. If you shut it off and let it sit for a couple of hours, it will take an hour or so to start again. Just weird.

[dickcdawg]

Check the water temp sensor at back of head - the one that signals the ECU that engine is at proper operating temp.  Service manual has the spec for testing resistance on the sensor.  They do go bad, and results in the kind of poor running symptoms you describe.

Dick H.

'92 Miata

#39 ITA - SeDiv

[Bench Racer]

I had an issue a while back, checked all sorts of stuff including the TPS per the FSM, checked ok to specs. Disconnected the TPS and issue gone. Installed new TPS, no issue since. 

[Dave D.]

This totally sounds like something is getting heat soaked, so it takes an hour of driving to get it to happen. Drive until it happens,how hot is the AFM to the touch? Take the cover off the AFM( the black plastic one)drive until hot and acting up, get out and see if the AFM door is swinging freely. I had a customer car car act up and the cold air intake got rainwater into the AFM and corrosion caused the door to bind and not move freely with airflow.

[Bill_LCFL]

Update. got my hands on a used AFM. Owner is doing a turbo install and said it was working fine. I put it in and no chage. About an hour and it started hesitating again. it is getting worse. Now it bucks and lurches. The frequency of loss is faster and shorter. But still like going to idle then back to what ever rpm it was at. I pulled over, disconnected the tps and it made no difference at all. i can smell a rick condition or fuel when this is happening. I was going to check the rear thermo sensor but it looks like it would be easier to take the motor out to check it. Is there another way to check it? Any ideas on how to diagnose this? No codes on CEL.

[Bill_LCFL]

Updated video. The throttle is being held steady. I disconnected the TPS and it made no difference.

[Jason J Ball]

I would also check your Idle Control Bypass Valve. This lets air bypass the throttle body when idling and is controlled by water temperature, if it not working properly, the engine will be getting too much air. You could pull the plugs and see if they are running hot (they will be white versus a normal black). 

[AW33COM]

If you go up hill, it feels like a power loss. If you are starting from a stop, every time you shift, the car just idles for about 3-5 seconds, then takes off.

 

 

Fuel pickup problem?

[Brian129]

fuel tank venting? 

 

maybe a new gas cap,  back in 91 the ecm did not pick up if the cap was bad.

[Bill_LCFL]

fuel tank venting? 
 
maybe a new gas cap,  back in 91 the ecm did not pick up if the cap was bad.

Is it a lack of venting or too much?

[Brian129]

I'm thinking lack of. After an hour of run time you might be sucking a vacuum in the tank.

Just a guess. In motorcycles it is very common to see that issue with a running issue after a decent run.

[Bill_LCFL]

I'm thinking lack of. After an hour of run time you might be sucking a vacuum in the tank.

Just a guess. In motorcycles it is very common to see that issue with a running issue after a decent run.

Hmm. You know, when I take the cap off to put gas in, the vacuum rush of air is like no other vehicle I have had. How easy would that be.

[Bill_LCFL]

Gas cap didn't work. Will try CAS cool off next