Caveat: I'm using custom fuel injection (Microsquirt)

So this weekend my idle started hunting. I was pretty sure I had sprung a leak somewhere that was opening and closing a miniscule amount while the car vibrated at idle.

My idle would go from 1000 RPM (where I have it set) to about 1900 RPM for a few seconds and then settle back down again. Whatever, drove the car for 300 miles anyway. Only time when it became an issue was at stop lights and trying to get through the free bridge traffic jam between NJ and PA - and then it was just an added annoyance.

When I got home I did not find anything obviously wrong. I plugged my laptop in to see if my IAC had gone out of compliance. It was fine but I noted my "coolant" temps were all over the place.

Coolant sensor for me is the stock location head temp sensor, plugged into my ECU.

Are these things known for failing? Mine was giving readings with a 100+ degree swings every few seconds (I'm guessing cutting out), and my idle air valve was opening and closing trying to keep up, hence the idle hunting.

Zach

Could be distributor advance mechanism malfunctioning?

I don't have a distributor. Crank fire.

In this case, I can pretty much narrow the hunt to the sender, the wires, or the connectors, as I was watching the garbage temp data being sent to the ECU over my laptop. The ECU uses that temp data to run the idle air control valve which is the prime driver for idle speeds - assuming you don't have a leak.

Zach

I wonder if it could be a weak connection? It's just measuring resistance on that wire...

The 914 engine will generally hunt at idle when it's too lean.

Sounds like you're dealing with aftermarket fuel injection ("crank fire", IAC, ECU, "computer"). As such, you're very likely on your own as to the cause of this condition, especially without more information on what you're running.

GA

Yes

Just re-read your first sentence in the original post (was it there originally?) Suggest you will have better support on the Megasquirt support forums:

https://www.msextra.com/forums/

Ha! My first post has remained un-edited.
Usually as soon as I mention Aircooled on the MSExtra forums people run and hide. Since I think my issue is with a 914 part, I figured I'd have better luck here.

Zach

You're not gonna like this but...

There's only a handful of us running MS systems, and each one is bespoke. For one simple example, we have no idea what CHT you're using, so asking on this board "are the CHTs subject to failure" is pointless (or it gets you responses from guys who think you're running the stock CHT sensor, which actually won't work on MS unless you have some kind of pull-up resistor circuit on it). We don't know what you have, so we have no idea if that is subject to common failure.

For another example, I'm not running an idle air valve controller with my self-designed Microsquirt system (quite successfully, I might add) so I have no idea what you're using or how you have it set up or are controlling it.

I personally spent a lot of brain time thinking through and building my design and I know it well, but I don't know yours. And I can't spend time learning your design so I can guess over the Internet if your CHT is the problem.

Further, probably none of us here is qualified to review your Megasquirt logs and see if maybe something is borked.

Did you buy this as a bolt-on system from someone (DubShop?) You can ask for help there.

Nope, we treaded off the beaten path with Mega/Micro. Lack of support from OE boards - or limited support from the aftermarket boards - is what we have to accept on these systems. Maybe that'll change over time if more 914 owners install these systems?

So I strongly encourage you to hit the Megasquirt support forums. Those are the MicroMega brainpowers, and the primary support system for these DIY EFIs. I've been there often (sometimes daily, reviewing posts) and have gotten decent support.

GA

Edit: If you are running the stock sensor in the stock location, switch over to one of these: https://thedubshop.com/modified-coolant-temp-sensor-package/

Alternatively, switch to a K thermocoupler under the #3 spark plug - much more useful information for engine safety - and port it through this device: https://www.adafruit.com/product/1778

There's a FB group for aircooled EFI stuff. Mario from The Dub Shop started it, and is fairly active there. If he doesn't have an idea or two, I bet somebody else will.

FB Aircooled EFI

If your temp sensor is bouncing 100 degrees that would cause lean issues and your computer is probably doing weird stuff to compensate. Not sure about all the kevetching about microsquirt vs whatever. You can still troubleshoot it. My other car has a fuel injection system that is as opaque as you can get. We still figure it out.

Pull that cht and put it in in some heating water and watch the output.

Do you know which way the cht runs on your system? Is it always getting that info or is it a greater than x kind of deal?

I should already know this, but I forget, are you just running a stock d-jet single wire head temp sensor for coolant temp on your MS setup?

100 degree sensor reading swings are for sure a problem, may not be your only problem though and while it may be a bad sensor, if you are using the single wire d-jet sensor it could also be a grounding issue or something causing a lot of noise on the ground path. After setting up megasquirt just about every possible different way on 914s I find to have the most reliable/cleanest signal you want to be sure all sensors are grounded back though the MS sensor ground path and not directly to engine/battery ground. I have switched from using stock CHTs to Marios temp sensors as they are 2 wire, use a standard GM curve, and are WAY easier to install.

https://thedubshop.com/fuel-injection/sensors/


Chasing idle hunting with Megasquirt isnt always as straight forward as it is with carbs or D-jet because as your RPM and manifold vacuum change with the hunting the MS starts pulling values from various VE bins, and if your temp sensor has huge swings in its reading then it could be kicking it back into warmup enrichment mode as well.

Shoot me some logs via email and ill take a look.

I bought one of the DubShop units.

I thought I was using the stock head temp sensor, but it appears to be one that has been modified (and I now remember Mark saying that he had made several modified ones). Its a 2wire unit that has been potted into the stock sensor housing.

I'm going to just pull it and replace it with the DubShop sensor when I have a spare moment. If the sensor is to blame (as it seems to be) it should be a quick fix. If not, I'll get deeper into it later.

Not going to lie, I've been busier lately than a stump tailed cow in fly season. Maybe next week I'll have time. Work. Cubs. Boy Scouts. Kids music lessons. and trying to get the bus repainted before it gets too cold to do so... Too busy.

Not sure what Mark was using in his sensors but if they had a custom curve you may need to change the sensor calibration table back to stock on the MS when using marios sensor.

I just use one of his standard sensors (not the larger type 4 sensor) and attach it with an engine tin screw. Way easier then trying to get to the stock sensor location.

Oh I for sure will.
The dubshop sensors come with the three point calibration numbers, and I'll put those into Tuner Studio.

Looking at the engine, looks like I have to remove the intake runners to get to the sensor bung, which means I'll have a bit of disassembly to do. Maybe sunday when I get back from camping with the Cubs in Gettysburg.

Zach

As James said, just mount it on one of the screws that secures the engine tin to the head. I use the one just above the intake port. That works perfectly fine for warm-up.

Then just leave the old CHT sensor in place until the next time you have that stuff apart and can get to it easily. - GA

Way back when, I used one of his sensors but it was for my stock D-jet car. So as I recall the conversation with him the discussion was about the heat soak and sensitivity of the digital sensor and warm up. I used it as a test mule and frankly...it worked great. That was for a D-jet car...so dunno about a MS set up. It's just a
thermister and to my knowledge used the same rating as the stock CHT.

I know his kinda dropped off the map here so I couldn't supply any other info other than what I've wrote.

Good Luck.

Edit...I used it in the stock location BTW.

hunting is always leanness

I'll see what's in the bag - have not even opened it yet. But I think I got the larger T4 sensor. Dubshop was OOS for one of the sizes. I know this was just last week but OMG it seems like it was last year. Cubs/Scouts camping. An emergency appendectomy. And doing pads and rotor on Amy's car. All in one weekend.

I'm so looking forward to hunting season where my job is just sitting still all day listening to nature and waiting for a deer to walk by.

Zach

update to this thread.

The Dub Shop temp sensor fixed the issue!

At least I think it did. I installed the sensor on the #4 tin screw, and my warmup "coolant" temps firmed right up.

The plug was a bit fiddly, but only because I was not getting the pins seated all the way. Once that was resolved, I was getting the expected temps and all seems to be right with the world. Wanted to take it on a drive, but, well, Ian. Not as bad here in PA, but I've had steady rain for 2 days and its 11pm. No thanks. I let the car idle up in the garage though and aside from playing with some of the values in Tuner Studio, I was not getting the hunting idle at all that I was seeing on the drive back from Flanders.

Sometime this week I'll take it out for a proper drive and see what's up, but I expect my issue is resolved.

Zach

Awesome.