Hi all, new to this site and I hope this is a decent spot to lodge this book of a post.


Setup: '95 1.8 with stock head, running JRSC M45 w/ 62.5mm Pulley on it. Stock injectors, megasquirt DIYPNP2. AEM EGO Wideband. Running GM IAT sensor, no MAF. Throttle body is before supercharger, with IACV routed from post air filter (pre tb) all the way up to a dummy TB housing on the intake manifold. Lightweight alternator pulley, waterpump pulley, and flywheel (this drove some odd changes in the tuning from what my tuner informed me of). Had it tuned 2 summers ago on 93 octane and ran great aside from high oil consumption (reason I had the motor rebuilt). Never really had any issues with the car and have driven it spiritedly on the road for close to 2 years now, all on the same tune.


So the other day I took my freshly rebuilt motor out for its maiden voyage. All is well, keeping RPMs under 5k and giving it around 50-60% throttle tops just to get some heat into the rings.

After a few minutes, upon coming to a stop sign I put it in neutral, and it dies. This was an issue I had originally when I got the car, as it had only a base map on it. I thought, that's odd, maybe I just need to adjust the overrun fuel cut to a higher RPM. Whatever - I restart it and keep going. This happens 2 more times.

After the second dying of the motor, I am accelerating through the gears putting some load on the engine, when suddenly after shifting (& letting off the throttle), I hear a metallic noise from my supercharger. It's a similar noise I've hear when listening to the car idle (rotors contact on the housing, I imagine. I can hear it when I stick my ear on the air filter). It's done this since I had the car, but never at this noise level, and never when I was actually driving it. If I throttled more, the noise would go away. However, I also notice at this point that with my foot off the pedal, the car is still accelerating and making power. I immediately step on the brake, which does slow me down eventually to a point where I pull off the side of the road. At this point, my idle is much higher than expected (~1400 RPM), and the supercharger is making that metallic rubbing noise that is amplified compared to normal. I pop the hood with it running, and notice my bypass valve is about half way open, and is bobbing up and down slightly (implying a change in vacuum). I look at my throttle, which is closed all the way. I try to look for any other obvious failures, and see nothing. I decide to blip the throttle, which in turn causes the engine to "run away". I blipped it maybe 15% open, and the engine goes all the way up to like 6.5k RPM and holds it until i go to shut the car off. I try to unhook the supercharger bypass hose and see if something was blocking it or anything looked odd, but found nothing. I restart the car, and it idles normally around 1k. No weird supercharger noise, like every other time I've started it in the last 2 years.


I think, weird. Let's get limp this thing back home (of course this happens when I'm the furthest away on my planned route). the second I shift out of 1st gear (letting off the throttle...) it repeats what had just happened. I decide I'm gonna try and wing it home, so I go to shift up - which causes my RPMs to jump again (on its own!!).

I decide to dump the clutch and get it into 5th, at which point it feels like I have cruise control because I'm doing 55 with my foot off the pedal.


Eventually I make it home and try to look again while it's running. I see on my laptop that my coolant temp is higher than normal, so I try to make it quick. I blip the throttle and it runs away again (~6.5k RPM). I shut it off and push it in the garage.

upon trying to restart it ~10 minutes later, I cannot get it to restart. It will crank and sputter one time, then just crank with no fire.



Initial thoughts are IACV/Vacuum leak. However, not being able to start the car, I just leave it alone for the night. I retightened some air/vacuum connections and checked the IACV connector and just kinda fiddled around with stuff hoping it would somehow fix it..


This morning I went for another shot at it. Started up fine. No weird nosies. Drove it around for ~10 minutes just doing some more break in pulls and enjoying it. Ran just like it had the last 2 years I had it. I went ahead and parked it as I just did this on a break in between meetings.


Puzzled, I wonder if this was some weird one off thing.


Then, this evening I had more time and went for a longer drive. I stayed close to my house this time in case the run away situation appeared again. Once again, it runs great with no weird noises and I get some good pulls in. Then, when going to turn around at a stop sign, the car NEARLY dies on me (like seriously 100 RPM). I do another pull on my way back, and as I pass my house, I hear the dreaded supercharger noise, followed by an engine that is running without my input. I slow down enough and limp it back into the drive way. RPMs are higher this time, around 1600. This time I decide not to blip the throttle and instead pull off the hose to the IACV and block it with my palm. No change in RPM whatsoever. Kinda rules out my thoughts on IACV being bad..


I tried my best to comb through the datalogs, but I'll be honest I'm not quite sure what to look for.


Things I would like to point out:

-Every time this has happened was after letting off the throttle.

-When this runaway situation occurs, my MAP is quite a bit higher than regular idle (75-80 kPa, vs 43-50 kPa) - I have now also learned that it seems I am pulling low vacuum at idle.....

-It seems my TPS is working correctly. As you can see in the logs, my TPS reads - 0% , but the MAP stays high, and the car accelerates

-I also noticed that my spark advance drops to like -1 degrees, whereas on my normal idle it is 15-18 degrees - from what I've read this also will decrease vacuum. (is the change in spark advance causing this increased manifold pressure??!)

-When this run away situation occurs, my coolant temperature is also notably higher (~185+ compared to under 170 normally) . I have a 165* thermostat in the vehicle.

-AFRs seem to be very stable during this "runaway" idle. Holds like 14.0-14.5 very consistently


attached is my current tune - changesToOverrun startRPM (After adjusting the overrun RPM) - ignition timing is weird at the bottom because I'm a child and like the pops when shifting. It's been this way for a little over a year

firstDriveWeirdRPMs datalog - which covers day one of driving with the runaway event (around 850 seconds for the initial runaway event)

coldstartEvening datalog - which covers day 2 of driving with the runaway event (not the quick morning drive) (around 790 seconds for the runaway event)

I found in both logs this issue occurs after ~800 seconds into the log (which also happens to be run time), which I find super strange.

the evening datalog is a bit less messy/shorter IMO, but I thought both would be useful if someone is really good with this stuff.


I really appreciate any guidance. I am planning to try and check for vacuum leaks with water tomorrow as I really do not care to catch this thing on fire. I'm super lost as to where the engine is drawing so much air from (if I am interpreting this correctly). I thought it would be the IACV, but with my blocking it off with my palm, I don't know where it's getting air from..

 

I didn’t read the whole post. Sounds like multiple issues, but rule out the physical ones first. if you’re letting off the gas but still building power, that is high priority fix to avoid injury to yourself and others. Two possibilities are:

1. Your bypass valve is torn, and locking he SC in boost. Check to see if it’s torn.

2. your throttle cable could be binding or not tracking exactly in the throttle wheel. This can accidentally keep the throttle plate open. The throttle cable guide that mounts under the M45 can work itself loose due to vibration/heat. A rare situation, but easy to miss when you’re getting the engine all tied up. Have someone push the throttle and ensure it’s positioned perfectly in track. Then try stabbing the throttle and blipping it for the same test. the throttle wheel can sometimes back faster than retracting the cable causing a scenario with loose slack.