UPDATE 25

Tested both boosters. The original one, now removed, loses vacuum slowly. From a vacuum of 22 in Hg, it slowly drops to 8 in 5 minutes. The new one, currently installed, goes from 22 to 18.5 in 5 minutes and probably 1 in Hg is due to a very small leak in the test rig. Idle goes from 800 when just started to 600 and then 400 when fully warm. At that time accelerating and letting go stalls on the way back to idle. It seems a little better than before, but that may be because the weather is colder.

Latest vacuum test: After disconnecting the brake booster and putting the vacuum gauge at that end of the hose I get a solid 20-21 in Hg. Reconnecting the brake booster and measuring at the "purge control solenoid valve" I get 13 in Hg. Very strange. There is a very small hose: rubber-metal-rubber that goes from the purge control solenoid valve to the throttle body. I suppose I could test the vacuum right at the throttle body, but that hose is probably not bad. If it were it should affect the vacuum when measured at the brake booster.

It appears that the old brake booster was losing vacuum more quickly, but probably wasn't the main cause of the problem.

I'm now at a loss. What else could it be? I've already replaced every electrical part. Suggestions welcome.

 

Check the hose clamp on the brake booster side. If it's not tight enough, this could cause your issue. If that doesn't work, I'd say I agree that the new booster is defective. My next question would be, did you supply the part to the mechanic? If he supplied it, it should be under warranty and I would fight to have it replaced. If not, well, you've got three options.

(1): Pull the new part, send it in for warranty replacement, and either DIY the replacement or pay to have it installed.

(2): Same as above except return it and get your money back. Hire a mechanic to replace it again but this time with a parts/labor warranty guarantee due to them sourcing the part.

(3): Park the Monty in the shade for awhile and revisit at a later time. Sometimes the ole hamster and wheel upstairs gets exhausted and will be rejuvenated with a short break.

 

Another thing you can do to rule out a bad brake booster is to disconnect the brake booster hose. Plug the engine side of thr hose so you have no air leak. Run the engine and check your idle speed and fuel trims. If engine runs good or better with that hose disconnected, you got a defective booster. If not, the issue is somewhere else.

 

That's what I did in UPDATE 25 above. I put the vacuum gauge at the disconnected brake booster hose and got solid 20-21 vacuum. Of course without brakes I couldn't drive the car around to see how the engine ran, and I'm not sure that the computer has adapted to the good vacuum condition in such a short time. In any case, I don't see the normal behavior: 2000 RPMs as soon as I start - it only goes to 800, the misfires are still there and when it warms up it still drops to 400 RPMs and eventually stalls after accelerating and lifting the foot.

I'm thinking that the difference in vacuum that I see between the brake booster hose and the purge control solenoid valve hose might be due (at least in part) to the difference in diameter of the hoses. The latter is tiny in comparison. I'll try next to put the vacuum gauge in line with the connected brake booster now that I have some additional hose adapters.

I'm thinking that it's conceivable that it's something else entirely.

 

Check for intake and plenum gasket leaks. Have you replaced the injector o-rings? If you have vacuum actuated cruise control, check the hose(s) and diaphragm. Have you check the PCV or EGR valves? Just a few ideas if you're wanting them.

 

UPDATE 26

Made a T setup of hoses and adapters for the vacuum gauge. Tested it separately to make sure it didn't have any leaks. It kept 22 in HG for at least 5 minutes. Installed it between the brake booster and the hose that plugs into it.

Started the engine. At first the vacuum was low but it soon got to 19.5. As I accelerated intermittently and the engine warmed up, it went to 20 and then 21. When it reached the temperature at which the purge control solenoid valve opens, it dropped a little but was soon back up to 20 and then 21. When I turned the engine off, the vacuum remained solid at 21 for 15 minutes.

During all this time, idle was 800, then 600, then 400. When fully warm, at 400 idle, racing the engine and letting go would almost stall (charging light flickered on) and recover, until it didn't.

I have not replaced the injector O-rings. Wouldn't a failure there be reflected in abnormal vacuum? I don't have cruise control or EGR. I have not checked the PCV valve, but wouldn't that failure also be reflected in low vacuum?

So, it seems the vacuum is good. When I tested the new O2 sensor (see the post of Aug 21, 2025 4:12 PM) the traces wouldn't budge from 0 V implying a lean condition. What else could cause a lean condition? The computer doesn't report any error codes. Maybe it's time for a trip to the dealer...

 

Fuel pump failure?

 

The fuel pump was replaced (along with the fuel filter, strainer and pressure hose) when I had the emergency of running out of gas (but I didn't know it then). I had to be towed and all those repairs were unnecessary. $1100.

The engine has plenty of power, it accelerates well, the misfires now occur only at idle when cold, so it seems there's plenty of gas being delivered. Is it possible that the fuel pump pushes too much gas that can't be properly handled by the injectors? That seems strange. What seems more likely is that the computer is letting too much gas in because it thinks the engine runs lean. Maybe it is running lean, but why?

 

There should be a fuel pressure regulator on the fuel rail. When working properly it should bleed off excess fuel back to the tank. What you're experiencing does not look like fuel pressure regulator issue. It is a sensor data missmatch, or still some sort of vacuum leak.
You can see the O2 traces in your scope, right? Try looking for vaccum leak once again. Get portable propane bottle (blue one with nozzle). Run the engine at idle with your data stream going. Open the propane gas flow (slow, and no fire), stick the nozzle right up to all different areas of the intake, fuel rail, etc and watch fuel trims or the O2 traces for change. Try to pump some propane into the intake so that you can see the system react. It looks scarry, but it works better than carb cleaner method.

 

UPDATE 27

MAF Sensor. The manual only says that at 700 RPMs it should be generating 25-45 Hz, at 2000 RPMs: 85-105 Hz, and when racing the engine: increase., so I made some wires to put the scope on the various signals. This is from the schematic in the manual:





It appears that the WB and R pins are not connected. Put the probes on GY and P. These are the traces I saw. They certainly don't look right. Large DC offset and tiny signal.





I also tested pin RB (the heater). This is the trace I got:



All referenced to ground (pin B)

Does this look right? Can anybody confirm that these traces are not normal? I'd rather not buy a new MAF sensor if this one is working properly as it's quite expensive (~$150).

I was expecting to see something like what is displayed here:

https://autoditex.com/page/maf-lab-s...orms-68-1.html

for the 1992 Mitsubishi Eclipse (a bit down the page). Interestingly, the note says that at idle the frequency is about 30Hz, which matches what the manual says.

What would cause the signal to be stuck at 4V? Would cleaning the sensor with compressed air be advisable? Do I need to buy a new MAF sensor?

Thanks for reading this far.