replaced a bunch of stuff, now won't rev over 3k

[Nevin]

Ok, so you're wondering why I posted this here. The reason... it's not on the svx, it's on my other car, an 89 mazda mx-6 turbo.

Some background... I put a new head on the car with new valve seals, some porting, re-lapped valve seats, new HLA's, a bunch of stuff... Also put a new turbo in cause my old one was starting to smoke a bit. Then I also did larger injectors and a fuel pump just because I felt like going to town.

Anyway, got the car all back together and it went really well. Took it out around the block and it wouldn't run over 3k rpms. Well, that's not exactly accurate. Anytime you try to load it down OR rev it over approximately 3k it just stutters. It actually acts exactly like a 2-step rev limiter.

So far I have tried a different distributor, (it houses the crank sensor) new coil, re-checked my timing, air flow meter, cap and rotor, plugs, wires, knock sensor, and still no results. Even suspected my msd 6a, so I unhooked it as well and still nothing! Driving it around without going positive manifold pressure you would never notice a difference. My dad could drive it and he would never know anything is wrong. My spark (at idle anyway) jumps about 1/2" and my fuel pressure and vacuum are both good! Driving back and forth to work (car barely even warms up) it still gets about 18pmg, so I don't think I'm dumping in too much fuel or anything. It just seems like something is pulling spark out of it!!

Anyway, it's just frustrating, and I was just curious if any of you had any good ideas to share with me about what to check.

Computer has also been suggested, but I dunno... I have another I could swap, so I guess it'd be easy to check.

Just let me know if you have any ideas! Thanks!

-nevin

[oab_au]

Originally Posted by Nevin

Ok, so you're wondering why I posted this here. The reason... it's not on the svx, it's on my other car, an 89 mazda mx-6 turbo.

Some background... I put a new head on the car with new valve seals, some porting, re-lapped valve seats, new HLA's, a bunch of stuff... Also put a new turbo in cause my old one was starting to smoke a bit. Then I also did larger injectors and a fuel pump just because I felt like going to town.

Anyway, got the car all back together and it went really well. Took it out around the block and it wouldn't run over 3k rpms. Well, that's not exactly accurate. Anytime you try to load it down OR rev it over approximately 3k it just stutters. It actually acts exactly like a 2-step rev limiter.

So far I have tried a different distributor, (it houses the crank sensor) new coil, re-checked my timing, air flow meter, cap and rotor, plugs, wires, knock sensor, and still no results. Even suspected my msd 6a, so I unhooked it as well and still nothing! Driving it around without going positive manifold pressure you would never notice a difference. My dad could drive it and he would never know anything is wrong. My spark (at idle anyway) jumps about 1/2" and my fuel pressure and vacuum are both good! Driving back and forth to work (car barely even warms up) it still gets about 18pmg, so I don't think I'm dumping in too much fuel or anything. It just seems like something is pulling spark out of it!!

Anyway, it's just frustrating, and I was just curious if any of you had any good ideas to share with me about what to check.

Computer has also been suggested, but I dunno... I have another I could swap, so I guess it'd be easy to check.

Just let me know if you have any ideas! Thanks!

-nevin

Why the larger injectors?? Did you change any other settings to suit the larger injectors??

Harvey.

[Nevin]

Originally Posted by oab_au

Why the larger injectors?? Did you change any other settings to suit the larger injectors??

Harvey.

Why the larger injectors? Because the turbo is much larger than stock. Similar setups (with standalones) have given 350whp/450wtq on stock bottom ends on these cars, compared to 150whp/190wtq. And although I'm not going to be pushing the limits NEAR that much, I wanted my fuel to be able to keep up with my turbo's CFM. My shadetree mathematics would guesstimate my airflow and fuel to be suitable for about 270 whp and 350-ish wtq.

I have reduced the base fuel pressure accordigly with my AFPR regulator at 0 vac. to adjust for the larger injectors. Folks have been doing this for years in the mx6 community with good results. Not as good as standalones, but they certainly ge the job done. Plus since it's not smoking and I still have decent fuel economy, I suspect it's related to something else. I also installed a walbro 255 fuel pump to keep up with the larger flow requirements.

It just seems spark related... Like something is pulling spark out. What other sensors could do that if malfunctioning?

Also, I've purchased an AEM wideband since I wanted to use it to help tune the car anyway. I'll get it hooked up, and perhaps it can help shed some light on the matter.

[SVXRide]

Nevin,
I'd suggest you step back and look at how the car was running before all of the mods and how it's running now. Were there any issues other than a little smoking from the turbo? What rpm would it run up to previously without any issues? Once you've got your baseline established, start walking through your changes.
1. while the mods to the new head don't really point to the current inability to rev above 3k rpms, how about the way the head was installed? Were all the timing points on the cam gear lined up correctly with those on the belt/chain? (yes, I know you said you checked the timing )
2. Is the new turbo exactly the same as the one it replaced? Have you double checked the install to make sure you don't have any leaks while under boost? Does it have an internal or external wastegate and is it functioning correctly?
3. and so on....

It does seem like you've got a "under boost" issue...
-Bill

[Nevin]

Bill, I appreciate the input!

The issues of why I decided to go ahead and do all this stuff were as follows.

1. leaking valve seals made the car smoke badly if it had been sitting say... 4 or 5 days. Lucas oil stabilizer had helped, but it was just getting worse.

2. Under "high" boost conditions (14-16 psi) I would sometimes get some black smoke as well. After checking the turbo, I did find a bit of side to side play which would lead me to believe a thrust washer was going bad, and a seal was on it's way out. It was cheaper for me to just buy a new cheapo ebay turbo than have my old Garrett rebuilt, by about 3x as much! I purchased a turbo that was an upgrade, but also a direct replacement to the one I previously had.

3. Exhaust manifold gaskets started leaking (about a week before, this is what really made me decide to do it.) The cause of this was that I didn't mount a turbo pedestal to hold the weight of the turbo when I put the larger one on before, and the studs started to strip after a while. This caused the car to have an annoying puttering sound as well as being down on power from the decreased boost. This time I replaced with 10mm studs instead of 8mm, and it does seem a lot more secure. Also I fabricated a new pedestal to hold the weight of the turbo which seems to be working well.


Really though, the car was running just fine before. Didn't really ever miss, had gobs of torque, and got decent mileage. It never stuttered before, and responded very well when the turbo started spooling up, no bucking or missing.

But once I decided to tackle all these things, I just decided that I may as well do the injectors and the pump as well for good measure. I didn't want to have all that work and then have the thing go "kaboom." I would rather overbuild than cheap out.

I installed the wastegate and checked it's operation as well. My thoughts are, if the wastegate wasn't working it wouldn't pitter patter like it is, my guess is that it would just make as much boost as it could, in the case of a malfunctioning actuator. I hardly thing it would cause a "studdering."

These are just all my thoughts anyway. Id love to hear if you have any other advice.

I'm trying to go back and retrace all of what I did so I can maybe narrow it down a bit.

And really, thanks again. It's good to hear ideas from someone else with more experience than I have.

[Nevin]

Well, an interesting turn of events...

I got an AEM UEGO wideband o2 sensor about a month ago for this car knowing that I wanted to use it sooner or later to get a decent grasp on the fuel tuning. I didn't have anything to do tonight after work so I decided to put it on.

Everything went pretty smooth really. The install took about an hour. Really, these things are so idiot proof. A bit of welding, run two wires, mount your gauge and you're done.

Well, I got everything mounted and started the car up. The gauge read about 14.5 or so and kept creeping towards the rich side until it was off the scale rich. Hmm... "Me thinks this a wee bit rich" say I. I got out and back off the fuel a bit (with my aeromotive FPR) and got it to about 11.5 which is still a bit rich, but not bad for cold start enrichment. Pulled my base pressure at 0 vac to about 20. It had been around 30. 30 probably seems low, but ah HA!! remember those big injectors I put in? Bingo. Let's just say they were doing their job, and putting as much fuel in as they were being told. They were just being told too much!

So at this point I'm still thinking to myself that this probably hadn't solved the problem, just that the car was indeed running way rich. It was in the back of my head though before the install that this could easily help shed some light on what was going on inside the combustion chamber. Little did I know...

I get out on the highway to go home and I hear this high pitched whistle that I haven't heard my car make in a while. Well my my, the car is boosting like it should! And not stumbling! Imagine that. Drove it around a bit, came back and backed the fuel off a bit more and it seems to be pretty good now.

If I back it off much more the fuel won't atomize too well at idle and I think that would suffer a bit, plus I'd rather keep it a bit rich for when I'm under "pressure." The turbo was only running about 7psi, but it felt very torquey and had good pull at all rpms. I bought this turbo to be able to run 14-16 psi, and my mathematic guesstimates would put it around 300 crank horsepower at high boost. I think I'll keep it around 10psi or so except for the track though. This is, after all, my daily driver.


Anyway, I just thought I'd share.


So tonight was a good night.