So I've been chasing this issue with my car still, where it will surge pretty bad in the morning for the first half mile or so of my commute ( after a 2-3 minute warmup). The car will still surge when warm, it is just harder to feel. Also noted that it feels sloppy on the bottom end to me ( but still rips and holds 21+psi to redline).

I noticed that my stumbling is pretty much linked to a noticeable lack in vacuum. The car when cold will see about 10 in/hg or so, but after just a few short minutes of driving will sit at the nominal 17-20 or so. No obvious vacuum leaks, however I am not ruling out that there is a sneaky one hiding somewhere on me. Is this low vacuum while cold typical of these cars? ( 35-40 degree weather in the morning).

Also noteable is comparatively bad gas mileage. New plugs, walbro 255, tuned.

Any ideas? I was thoroughly convinced my upstream 02 was on the way out but at this point I am unsure. Would a shot factory cat cause a loss of vacuum?

 

It could depend on your mods.

By that I mean...
1. Engine's will suck vacuum when operating obviously. But having a turbine strapped to the engine, especially on a tuned car, is obviously designed to overcome vacuum and provide boost.

2. When your engine isn't warmed up, it may idle at a higher RPM (though it should be relatively constant). This higher RPM could be spooling your turbine enough to provide some additional psi to take away from the vacuum.

3. What exactly do you mean when you say "surging"?

4. What do you mean with poor fuel economy? Has it changed recently because of a problem you suggest you may be having, or did it change with the weather?

Colder air is significantly more dense than warmer air. More dense air, means your car needs more fuel to compensate. This happens on all modern cars, but on a turbocharged vehicle, the effects are significantly more noticeable because of course, you're compressing more air into the engine.

What type of changes in fuel economy have you noticed?

 

3-4mpg.

Surging as in like the way a car feels when it has water in the gas. Only this has gone on for months. Several fuel treatments ago.

Car is otherwise stock. Idle is noticeably choppy at times. It will idle fine, then you hear a burp every so often like the car is choking on fuel.

 

stock cams?
with stock cams I would say 20 +/-2 is normal.

 

Sorry to get all scientific on you... but 3-4mpg is meaningless unless you flip the fraction.

Are you going from like 20 to 17 or what? We need numbers.

I thought I read that you had done a boost leak test, but can't find it now. So...
1. Boost leak test.
2. What psi are you tuned at?
3. Are your plugs gapped correctly?
4. Did you purchase the correct plugs for your engine? Because not all Evos are created equal.

Most importantly... what CAUSED this problem to happen initially?

 

If I knew why the car was running like poo, I wouldn't be digging for solutions.

Car was tuned by STM so the basics are off the table. They put brand new BR8E's in it and gapped them. This surging deal is just a symptom that has been getting progressively worse over the past few months, and has now become more noticeable. No check engine light. They did the boost leak test, but all bets are off at this point.

When I say 2-4 mpg thats what it dropped from my average figure of 23/24 mostly highway. It is down around 20 or so now. Regardless, the point is its significant.

 

Stock cams. And yes it is around 17-20 or so when warm, but the issue is that I am seeing like 10 on initial startup in 40 degree weather, and it takes the car 2-3 minutes to start pulling the 17-20.

 

I agree that it's significant, but a drop from 30 to 27mpg is MUCH less significant than a drop from 20-17mpg because of the fraction.

I understand that you don't know what the problem is. I was asking WHY it started. You said you've noticed it getting progressively worse... is that based on temperature, or was there a mod you've done, or something.

The first thing that I might do is PM Emery on here, or give him a call (he does seem to hate non-verbal communication haha).

I would also suggest that you start by doing a boost leak test. They're simple enough to do, and I'd be happy to talk you through it if you can't find one of the threads on here explaining it.

Seals can expand and contract in temperature changes. So even a car that has just been checked could theoretically develop a leak. This seems most probably to me.

 

The reason you're seeing higher numbers when your engine is cold is because it's idling at a higher RPM. I would consider that normal.

The surging is not normal.

Do a boost leak test, and report your findings.

 

I don't doubt they would be able to get me straight if they had the car a few days, the issue is I am like 4 hours away and need the car to get to work all week. If this isn't cleared up by April I will just have to bring it back out there. I don't like harassing those guys any more than I need to, I know it is hard if not impossible to troubleshoot something this finicky over the phone.

Propilot enlighten me about this fraction thing you speak of? I must be missing the boat there.

 

After full warmup, the upstream 02 sensor is taking more than a second to switch voltage at idle, and even when you hold the throttle around 3 grand. I am holding off on installing the sensor until I check every vacuum line in the car one more time.