Engine warm up problem

[gdaybruce]

I have an irritating problem with my 1.8 Eunos and any suggestions on its cause would be welcome!

A while ago I suffered a split oil cooler hose – the one that goes from the back of the cylinder head to the oil cooler. There followed a very long story that I won’t bore you with but suffice to say that the car now has a reconditioned cylinder head, new water pump and, of course, cam belt.

Generally, the engine is now running very well except for one small issue: it starts from cold with no problem but after 30 – 60 seconds of running it starts to idle lumpily and any application of throttle cuts the engine stone dead. After another 30 secs or so it will restart and continue to idle lumpily but once there is enough heat in the coolant for the water temp gauge to register it smooths out and runs completely normally.

In my attempts to fix this I’ve:

- Replaced the coolant temp sensor at the back of the head,
- Confirmed that the Idle Air Control valve is operating
- Checked that the Throttle Position Sensor appears to be working
- Checked that the Mass Airflow Meter appears to be working

My toolkit doesn’t run to an ohmmeter so my “checks” amount to disconnecting and reconnecting the electrical plugs while the engine is ticking over and seeing if there is any effect, which there is.

There is a flow of coolant through the throttle body and presumably there is some kind of temperature sensor built into the IAC valve but I’m not sure how this works. It appears that if the valve is faulty it simply has to be replaced.

However, I’m really not sure where to go from here, unless I can find a known good IAC valve or TPS to fit and see if that fixes it. Has anyone come across this issue and can advise what’s likely to be wrong?

The good news is that the car is basically fine; I just have to leave it ticking over for a couple of minutes before driving away.

[MX-5 Lazza]

Have you gone through the full idle setting process? It could just need adjusting.

I believe the coolant flow through the TB is just to regulate the temperature in the TB to help stabilise the idle. Probably helps stop the butterfly freezing up too though that's not really an issue in this country.

[gdaybruce]

MX-5 Lazza said:

Have you gone through the full idle setting process? It could just need adjusting.

I believe the coolant flow through the TB is just to regulate the temperature in the TB to help stabilise the idle. Probably helps stop the butterfly freezing up too though that's not really an issue in this country.

Yes, I've reset the tickover with a jump wire connecting the grnd and 10 terminals in the diagnostic box. I had to slow it down a fair bit - it was ticking over at 1500rpm initially, but I had advanced the timing a few degrees from standard and the recon head had been resurfaced and might be running a marginally higher compression so perhaps it wasn't surprising that it neede some adjustment.

[MX-5 Lazza]

Any idea how much was taken off the head when it was skimmed? Could the cam timing be out? If much material was taken off they usually need vernier cams to get the cam timing right again. I don't know what the symptoms of that would be though.