lukec53

I just got my car running on my megasquirt today, but I cant seem to get a normal idle. It's an '88 turbo. My idle screw at the throttle is set all the way off.

At first I had my stock PWM idle valve hooked up, and when I started the car the RPM immediately shot up to 3000 and kept climbing untill I shut it off at about 3600. When I had the car running on the stock ECU, I had a similar problem and to fix it I just unhooked my idle valve and set the idle with the throttle screw. I tried this with the megasquirt, and it helped, but the rpm would start at 2000, climb to about 2350, have a AFR spike to about 16:1, then die out. I had to push the throttle to keep it running.

I adjusted the VE table and got it idling more steady, but it was still high. I added fuel and adjusted the spark timing to bring it down to a surprisingly smooth idle (for a rotary). The problem is, this idle is at 1500 rpm, and at an air fuel ratio of 10.2-10.8:1.... As soon as I let it get anywhere closer to stoich the RPM just rise until it hits richer boxes on the VE table.

There has to be something I'm doing wrong.. I'm new to tuning so to get the results I've gotten so far I've been researching lots and using intuition.

Any ideas of what could be causing this?

lukec53

Also, I just checked for vacuum leaks with throttle body cleaner (didn't have carb cleaner but its pretty much the same stuff I think)
I left it idling with the data logger going and sprayed cleaner at every possible place there could be a leak, and the rpm didn't budge.
Here's the log if it helps any.

Aaron Cake

iTrader: (1)

Which screw? The one on the BAC valve, or the hard idle screw on the throttle body?

A rotary, as long as it's not radically ported, should idle silky smooth at 750 RPM.

Is your cold start cam and thermowax still in place? Is there a coolant feed to it?

It sounds as though you either have a massive vacuum leak, or that the throttle plates are sticking open. Seems to be a common problem these days. Is the throttle cable too tight?

lukec53

I was referring to the hard idle screw on the throttle. My BAC valve screw is screwed in all the way (fully clockwise).


I'm running stock ports so that shouldn't be an issue :P

Technically my thermowax is still bolted to my throttle body (when I first did my TB mod I had the coolant lines still attached), but it is not doing anything. Cold start cam, extra plates, etc are all removed. The holes were sealed via welding.
Also, I had the car running with the current throttle body condition before I swapped to MegaSquirt, and steady idle at about 1000 RPM

I thought it would be a vacuum leak too, the symptoms fit perfect. If it is, its a very stealthy one, because I couldn't find anything with cleaner; I went all the way around the throttle body, upper intake manifold gasket, the various vacuum hose connections.
I double checked my throttle cable tension and backed it off so that I know there's a bit of slack there, and the problem didn't change. Also, when I play with the throttle by hand, it seems to snap back to fully closed (I can hear it thump when it springs back). I guess I could take everything apart and check out the seal of the throttle plates if I can't find anything else...

Aaron Cake

iTrader: (1)

Can you temporarily cap the hose that brings air to the BAC, just to make sure the BAC isn't randomly bypassing?

Stuck throttle bodies are something I've been running into more and more. Quite often it feels like it has closed, but if you then push on all the various levers, the idle drops back down.

Maybe the vacuum leak is very hidden. For example, something as crazy as the diaphram in the brake booster being bad.

lukec53

That's for sure! I covered the intake pipe completely, and the car kept running!

Also, I capped the bac valve and nothing changed. The same when I capped the hose to the brake booster.

I did however find a pretty good leak at my ACV block off plate (proves that rushed installations are a bad thing). I totally re did it yesterday, new gasket and used gasket sealer, and gave it over night for the sealer to really set. I'll let you know if that fixed it when I start it today.

lukec53

Ok so I've run the car a bit today and it's running a lot better. I've got an idle down lower now so I think that was my problem. It's still no 700 RPM, but im pretty happy with a 1000 RPM idle.
Thanks man and sorry for posting so much.. I'm learning lots though!

I came across a new problem now though :/ I hope these are going to stop soon... :P
When the car is idling, it starts up fast and idles smooth... but then after a while, the AFR shoots up very suddenly and it just die abruptly. I data logged a start-die run and found that right before it dies, my pulsewidth and duty cycle go to 0 for a split second. I have no idea what it could be caused from. I thought it could be my overrun fuel cut kicking in accidently, but I upped the RPM to 2000 and it still happened. I don't have overboost enabled.

What else could cause a split-second fuel cut?

I've attached a couple logs of this happening, just bring up PW and you can see what I'm talking about.
I'll probably just post this as a new thread but just in case you have an answer...

toplessFC3Sman

iTrader: (1)

I found that the throttle-stop screw for the secondaries was holding them open a little too much when I first went to MS too, so thats one more thing to check. In addition, I'm using the BAC valve for closed-loop idle control & wanted to give it a little more authority, so I found an appropriately sized small screw & used it to plug the hole in the primary plate, then ground it off so that it couldn't back out. If you do this, just make sure to use threadlocker or J-B weld or something to make sure it doesn't come loose & get injested.