Engi-ninja

First of all, I know this sounds like something that's been addressed many times before, but I've tried the fixes that I've seen on here, and none of them have worked so far. Also, a lot of the info about stumbling an warmup issues refer to MS tuning, but I have a stock ECU.

I have a stock 2003; stock engine, stock ECM, stock everything except the wheels and a short ram intake right now. I have a megasquirt, but Reverant has it currently, trying to make it play nicely with the VVTuner.

Anyway, the car starts and idles fine. It also runs fine as long as the temperature gauge is at zero. Once it has warmed up enough that the gauge starts moving, it starts experiencing little stumbles and surges at steady, light throttle between 2k-3k rpms. It seems worse in 4th and 5th gears. Once it warms up to steady state, the stumbles disappear suddenly. In fact, if it happens to be in the middle of a stumble when it reaches steady state temperature, it's like a switch goes off and the stumbles stop immediately.

I replaced the O2 sensor and ran seafoam through the intake and the gas tank, which didn't solve it. Then I found the various threads about the VTCS causing issues, and I thought for sure that was it because it sounded very similar to my issue! However, disconnecting and capping it actually made it much worse, so that it hesitated and stumbled at all rpms until it had warmed up.

I checked the connectors on the IAT and MAF, and they seem fine; I haven't taken the MAF out yet to see if there's anything gummed up inside it, but it seems like if there were, it would cause issues all the time, and not just during warm up, right?

Does anyone have any ideas on what else could be the issue? Maybe the ECU is malfunctioning and messing up the warmup fuel mixture? Any ideas would be appreciated.

Thanks!

18psi

iTrader: (76)

perhaps the solenoid for vtcs is messed up and causing them to partially open during warmup?

Engi-ninja

I had thought of that...I suppose I could replace that solenoid and see what happens. Does it seem weird that disconnecting the VTCS completely caused it to stumble and hesitate even more? I thought it was only for emissions?

18psi

iTrader: (76)

no. cold idle stability too. we just talked about this in another thread earlier today lol.

I'm not saying that's 100% it, but that's where I'd start.

Engi-ninja

Oh OK, haha. Well, it actually seemed to idle fine with it unplugged; it wasn't till I started driving that it seemed to make it worse. I'll look into getting another one though, and see if that helps. I wish I could just plug my mega squirt back in so I can see what's going on, but Rev's got it, and I'm not sure when it will be finished; I haven't heard from him in a while, lol.

Engi-ninja

Ok, so this is weird. Just for kicks, I disconnected the upstream O2 sensor completely this morning before driving to work to see if it made any difference; I immediately noticed 2 things:

1) The CEL did NOT come on.
2) The stumbling issue went away.

Any ideas on what could be causing this? Shouldn't the CEL come on if the O2 sensor is disconnected? I guess I can just leave it disconnected, but I don't want to kill my gas mileage by putting the ECU into "protective mode" or whatever it's called, where it makes the fuel mixture rich to protect against knocking.

Any thoughts?

Thanks!

18psi

iTrader: (76)

probably bad o2 then

Engi-ninja

I replaced the O2 sensor with a brand new one, and it made no difference.

18psi

iTrader: (76)

bad tps? since that dictates when the car goes into closed/open loop
I dunno. just guessing here

Engi-ninja

I guess that's possible; I wish I had my megasquirt, I could just look at the readout and see what it's doing. There doesn't seem to be any other issues with the throttle though; no lag on acceleration or anything.

Dustin1824

iTrader: (2)

I'm experiencing the same issues with my NB2. Probably VTCS getting stuck open. After warmup, motor feels fine. I have actually gotten a CEL a couple times for "Intake runner stuck open condition". I'm pretty sure its the VTCS.

The stumble isn't so bad, it's the surging that sucks. If it starts to surge, its so strong, not only would passengers notice, but other drivers will notice.

I have actually been experiencing it for so long now, I can drive it so that during warmup no surging is ever experienced. I can feel how the motor start to faintly stumble right before it surges, and that's when I shift to avoid the surging. Honestly this has helped me take it easy on the motor until it is warmed up. The car only sees like 15% throttle and no more than 2500 RPM before warmup.

I have a spare solenoid, I just need to go through the test procedure in the Mazda manual. The process seems very detailed, and will probably take an afternoon.

Hopefully the VTCS system is the answer. If not, I'm not sure where to go from there.

Engi-ninja

So did you try just disconnecting and capping the VTCS solenoid? It seems like if that's the issue, disconnecting it would have some effect on it, good or bad. Apparently it causes some rough running conditions for a few seconds when it's cold, but when I disconnected it, idled fine and just ran rough under load for about 5 minutes...so which doesn't sound like what it does for other people.

curly

iTrader: (25)

Are the valves normally open or closed? I could look at one of my 3 vtcs manifolds at home, but I'm at work right now. Just unplugging the solenoid could keep them closed and cause all sorts of unrelated problems above idle. Just unplugging the solenoid probably isn't the answer, you'd want to hold them open somehow too.

Engi-ninja

According to the various threads on here, the consensus seems to be that the vavles are spring loaded open, and the vacuum line closes them during cold starting conditions until it's warmed up or it goes above 3500 rpms. Disconnecting and capping the solenoid should leave them open all the time, which should eliminate any issues with them malfunctioning...but it didn't seem to work for me. I think I'm going to try disconnecting it again and leave it disconnected for longer, and see what happens.

Dustin1824

iTrader: (2)

I don't think keeping the butterflies open all of the time is the answer. Like you said, the ecu keeps them closed at low rpm until the motor is warmed up. We are only experiencing problems at low rpm, so therefore I think the ecu is expecting the butterflies closed, yet they are partially or fully opened. My CEL was for the butterflies being stuck open.

If keeping the butterflies open was the answer, I would just swap in my square top manifold and be done with it (no butterflies = open butterflies)

Seriously, if I have to take the intake manifold off to fix the issue, it will be too tempting to just swap on the square top and see what happens

18psi

iTrader: (76)

they are spring loaded open. vacuum pulls them closed.

I just thought of something: did your car just start doing this or did it always do it?

cause on my 01 I remember when it went back to stock I accidentally connected THE WRONG vacuum line to the vtcs actuator and it did bog/stumble/surge too.

quickly realized my boo boo and fixed it next day.

just a thought

there are multiple vacuum lines in that spot going from the charcoal cannister/mani/tb and its easy to hook em up wrong

Dustin1824

iTrader: (2)

For me, it always did it, at first it was barely noticeable. Over the last year it has gotten worse. It got noticeable worse after my first track day at Hallett last September.

One thing that I notice is that it definitely gets worse when it gets colder. I think my butterflies are just really gummed up from the stock EGR and PCV system, and the gunk is thicker when it is cold, so that VTCS actuator can't close the butterflies as well as when it is hot. It was the worst when I would cold start my car in the morning and it was about 24F outside a few weeks ago.

I checked all of my vacuum lines for leaks and such about 2 weeks ago, they still feel pretty elastic and pliable.

18 brings up a good point though, there's quite a bit of vacuum lines in this area. I have a hose diagram to share.



The diagram makes it seem much simpler than it does on the actual motor.

Engi-ninja

Mine started doing it this past winter; I hadn't noticed it before that. I think it was very subtle at first, and then got worse as it got colder, just like Dustin said. I had never messed with any of the vacuum hoses in that area, so I don't think it's possible that the wrong hose is connected.

The part that confuses me is, why does it run fine when it's really cold, but as the engine starts warming up, then it starts acting weird until it's fully warmed up? I'm in Indiana, so it's been frequently below 0F here this winter, and sometimes it wouldn't start stumbling until I'd been driving for like 5 minutes, because it took so long to warm up. That's in addition to letting the car sit there running for 10 minutes before I started driving.

Dustin, I just noticed that you bought Vlad's spare VTCS solenoid...I'm guessing that didn't change anything?

Reverant finally got my MS working, so when it gets here I think I'll be able to learn a lot about what's going on when it does that stumble. I'll post some logs and see if we can find anything interesting.

patsmx5

iTrader: (16)

My guess is vacuum leak. I just fought some weird problems with my 99' on stock ecu and almost every problem ended up being a vacuum leak!

Dustin1824

iTrader: (2)

Since your going MS, can't you just wire the VTCS butterflies open or remove them altogether, and tune it accordingly? I am only fixing mine because I have to pass emissions, and even if I buy a MS, I will still have to return the car back to stock and pass emissions every year.

Yeah I bought Vlad's spare VTCS solenoid, but I still need to install it.

I am going to replace all of my vacuum hoses if I test everything and it all checks out. At least for me, I don't think that;s an issue, my lines feel pretty good, but it won't hurt to replace them.