Hi all,

I’ve got a 93 6spd car that I recently acquired, and when I bought the car it had a wandering idle that had me believing it was a vacuum leak, and a bit of a stumble that seemed like old fuel. I ran the tank empty of gas, and filled up with a cab of Seafoam and fresh 93 and both issues went away for nearly 2k miles. I ran it down to the reserve once, and now when I start the car and the oil temp is cold I get both the surging idle and a terrible stumble/misfire that trips the Service Engine Soon light. It’s also started to burble and pop from the mid muffler on decel. I have since checked my plugs and throttle body, which look fine, and replaced the fuel filter and injectors to no effect. Next on my list is gonna be the coolant temp sensor and IAC stuff. Any tips/direction would be welcome!

 

Check your fuel pressure first (especially when this is happening).

It sounds like there might be gunk in the tank, or something wrong with the sock or a failing fuel pump.

If fuel pressure is fine then it’s on to spark etc…

You are stating this is a cold-only issue?

 

Would an issue with the optispark do this?

 

I would think it would be reverse (issue when hot not cold), but OP needs to respond with more info. At this point could be anything…fuel, ICM, coil, optispark, FPR, spark plug wires, CTS, EGR etc…. If it is truly better warm then I’d suspect/check the CTS first by verifying the analog and digital readouts of temp.

 

I'm gonna borrow a fuel pressure gauge this weekend and check the pressure out, but yes, the poor running only happens when the engine temp is cold; as it heats up the issue subsides. Seems like the threshold is 190F on the oil temp. The car will hitch and stutter when it first starts, and if the car doesn't idle high on its own until it's warm the idle will drop low and the tach starts bouncing all over the gauge, misfiring and triggering the service light. A restart will make that condition go away if the idle is held above 1000rpm until the car is at operating temp.

I was thinking crap in the fuel system, hence the filter/injector replacement. Maybe I've cooked the pump, the pressure will tell me if that's the issue I suppose.

The readout on my digital cluster for the coolant temp seems to be accurate, it's tracking about 15deg above oil temps most of the time unless I'm on the highway, then they're pretty similar. Dial gauge in the cluster works intermittently.

For reference, the PO stated a recent rebuild (and evidence points to this being the case, but can't speak to the quality aside from the engine working before this issue. Shop stated a rebuild due to a spun bearing with OE equivalent parts) and the behavior doesn't seem to scream opti from what I understand, but I'm new to this platform.

 

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Optisparks usually have issues when they get hot, but possible. Need to eliminate other suspects/try to hone in on fuel or spark. If it’s fine when warm I would doubt a fuel delivery issue…but good to verify with the pressure gauge. Also check your FPR and make sure there is no gas in the vacuum line.

I just realized something, you said it trips the SES but did not list the codes. Can you pull codes please and post (there should be H or history codes)? It might point to the culprit (such as lost resolution pulse from OPTI).

 

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Ok, so I've done a little swapping with some of my buddy's known good parts and some new ones. Checked the coil on another recommendation and replaced the coolant temp sensor by the water pump to no effect.

Codes shown are H74, H28, H32, and H36.