MS "pop" + Boost Gauge = Crazy Problem
 

Ok, I've got a really strange problem and I've ran out of ways to try and fix it.

I've got the same "pop" from my MS as the one discussed in this thread:
https://www.miataturbo.net/megasquirt-18/pop-when-turning-ignition-not-cranking-burned-fuel-smell-26489/

Now, I've been able to minimize it by holding the accelerator down for a few seconds before I turn the car on.

So a few weeks ago I noticed my mechanical boost gauge was only showing about 15 inches of vacuum at idle. I connected the laptop to the megasquirt which showed 20 inches of vacuum. I then used a vacuum tester gauge which confirmed the numbers from the MS. So I tried moving my vacuum line to the port beside the MS's port but it still showed 15 inches at idle and suddenly it was showing that I was running 10lbs of boost (not possible with my setup).

So a friend was nice enough to loan me his electrical boost gauge but it is showing the same numbers as my mechanical was. Until Saturday night...

That night I forgot to hold down the accelerator and the car "popped" again. When I drove it home the boost gauge was correct. But the next time I did the pedal trick to eliminate the pop the gauge was wrong.

So it seems the "pop" somehow makes my boost gauge read correctly. I understand how the sudden pressure wave could cause the gauge to act funky but I don't get how it fixes it or even modifies it's readings after the event.

If anyone has any clue as to something I could try please let me know. I've kinda ran out of logical explanations for this. It would seem like the mechanical gauge would read pressure compared to ambient regardless of any strange "startup measurement." It also seems weird that this only affects gauges and not the internal map sensor in the MS or the vacuum tester gauge. Although I think the accuracy of the MS could be explained by the length of vacuum hose from the manifold to MS letting some of the pressure wave dissipate.

Thanks!

Sounds like you need to do joes spark mod to the MS if you haven't yet. The pops will go away.

Right now I don't want the pop to go away. I need it to happen so my gauge reads correctly.

Where is it pulling its reading from?

I've tried three different ports from around the manifold. Right now it's on the unused port on top right after the TB.

your shit might just not be calibrated correctly

Did you test both guages with the same line connecting to manifold and in all three spots? Check the line to see if its collapsed in any spot, esp any spot it has to bend around.

New vacuum hose and I only checked two ports with both gauges but at this point the third would be purely academic. Remember, the pop FIXES the gauge. Not having the pop causes the gauge to read wrong.

Update... still havent solved this.

I installed a restrictor in the boost line but nothing has changed. It seems that the gauge (both mechanical and electrical) are simply not detecting the correct ambient pressure. While I'm short ~4" vacuum I'm showing boost ~3lbs higher than actual. Does anyone know what would cause a system to do this?

Remember that I've replaced all the hoses and tried different ports on the manifold with no change.

Sounds like your gauge is a piece of shit.

New gauge:$36.95
Brass Tee, 1/8: NPT to hose-barb, compression fittings and 1/8" tube at ACE hardware, installed inline with brake booster hose prior to checkvalve: ~$20.
Perez Spark mod: $5.

Not sucking: priceless.

Well I've tried two gauges. The first was a cheapo autometer mechanical, the one in the car now is a nice electric, full sweep, Cobalt. Both do the exact same thing.

Joe, I don't know if you've had a chance to read the original post but the MS "pop" on startup actually fixes it.

The electric gauge is not a piece of shit. It read correctly when compared to my old Link and when I had a Hydra. Nothing wrong with it.

Now moving the source to the booster line may be a good idea, that's where I had mine.

I'll definitely try that this weekend. Perhaps having it farther away will help, except that the pop fixes it, which I don't want to loose.

Just to add more data: Yesterday I was driving it home (still inaccurate) and I let the car engine break while going down a long offramp. It seemed to pull it back in line by ~1 1/2" vacuum. I've yet to be able to connect this with an actual fix or replicate it.

Yeah, I read the whole thread. The fact of the pop fixing it is what makes me smell a rat.


There are too damned many potential vacuum sources on the manifold, and even I'm not certain as to where every single one of them actually sources from. One thing that's certain is that the big one on the back which feeds the brake booster is an absolutely 100% reliable, restriction-free reference point, provided you tap it before the check valve, which is a lump halfway through the rubber hose that connects to the brake booster itself.

If you take all of your manifold pressure references from that line, and your gauges / sensors are actually working properly, you will get an accurate reading. End of story.