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Old 05-14-2015, 06:47 PM   #1
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Default 1.6 to 1.8 swap idle and performance issues

Hey guys,

I've recently put a 1.8 engine from a '00 Miata to my '90 Miata. I used the 1.6 ECU, wiring harness and the 1.6 throttle body with IAC and TPS. We fabricated the adapter to put a 1.6 throttle body to a 1.8 intake manifold. On the first start, idle was bumping from 1.8k RPM to 2.2k RPM. So we used a simple gas regulator on the IAC hose that comes from the intake hose to the IAC and that solved the problem for idle. But the car didn't drive well (even without it with the big idle jumping) - until 5k RPM it was very slow, it consumed a lot of fuel (15ltr/100km).

Then my "guy" from the service, where we did the swap, did something with the timing and it started running like hell, fuel consumption went down to 6ltr/100km on the highway. But the idle stands still after running for a bit at 1.8k RPM.

Before that timing change, I did close the VICS valve with zip ties ( I know, ghetto, but it worked) and it became a little bit better on the low RPM's, and nothing changed on the high RPM's. Also, no black/blue-ish smoke on very high RPM's.

After the timing it started spewing black/blue-ish smoke and it doesn't accelerate as well on high RPM's as it does on low RPM's.

So I have two questions - is there any possibility to bolt a 1.8 throttle body to the 1.8 manifold but using the 1.6 TPS? And what can I try to do about the bad idle? My "guy" says if we set the timing back according to AutoData, the idle will be good, but the fuel consumption and acceleration will be back to being bad.

I'm a little stumped right now :(

P.S. Megasquirt is not an option right now, because of my finances.
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Old 05-14-2015, 07:06 PM   #2
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Interesting; certainly assuming you're using the CAS from the 90, I suppose it would run. The engine control would be rather different between the two in a lot of ways...

The idle control is likely radically different between the 90 and the 00. If I were in your ill-placed situation, I would disconnect the idle valve wire (maybe even cap off the plumbing) and just control the idle with the idle screw.
I ran my old engine without an idle valve and it worked great with the existing coolant valve and idle screw.

And set your base timing back to where the 00 engine is supposed to be.

Edit: I hesitated to point out the obvious about your fueling being off in any open loop condition. On second thought if you plan to continue driving the car as is it would be worth your while to at least get a wideband O2 sensor to make sure your open loop (WOT/heavy load/etc) operation.
Also, double check your mechanical timing.

Last edited by leboeuf; 05-14-2015 at 08:52 PM.
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