Engine bogging down with any swift throttle change

[JustJeff]

I also put this in my build thread, but that thing has gotten so long and often times mundane that I doubt a lot of people are reading it.

Long story short, engine is up and running but...

But, it still has the same bad bogging and event stalling if the throttle goes up with much of any quickness. Even bumping the RPMS from idle to 2k will cause it to bog and stall. It was doing this before rebuild and I thought I had found the cause in that my charge pipe to cold side coupler was splitting as I was dismantling things for teardown. I thought it had a small tear and my problem was a vacuum and pressure leak. That is not the case.

I'm still bleeding the coolant so I haven't had much time to diagnose anything. What I did notice is that the engine idles better and has less of a problem when vacuum hose that feeds my OEM boost sensor as well as my boost gauge is off the UIM. I use the nipple below the BAC with a tee to feed both OEM boost sensor as well as my aftermarket boost gauge.

When time permits I'll start testing TPS, OEM boost sensor, AFM, etc. Any thoughts on this would be appreciated.

[RETed]

Check TPS first...
Make sure it "reads" linearly from rest (i.e. idle) all the way up to WOT.
Make sure max resistance isn't over 5.5k-ohms.

After checking the TPS, unplug the boost sensor electrical plug and see if this condition changes?

If those two things check out, I would try and eliminate the ignition (spark) side of things, since it's easier at this point...
Make sure all our spark plugs are firing, etc.


-Ted

[Rotary Evolution]

and i wouldn't worry too much until the engine has at least a few hours on it, the idle has been reset, TPS adjusted and base timing set. if it is still doing it do a TPS sweep to check for dead spots and a vacuum leak test. low fuel pressure can also be a problem, as well as poor engine grounding.

[JustJeff]

One inaccuracy. I do not have my OEM pressure/boost sensor routed from the UIM nipple under the BAC. That nipple feeds both OEM BOV and AEM boost gauge. The OEM pressure sensor is routed to/from the cold side turbo splitter, which also is routed to my MBC and then from that to my wastegate.

What I noticed today is that idle jumps up about 500RPM when I pull the hose off the UIM to BOV and the idle evens out. Also if I pull the hose off the OEM pressure sensor or unplug the pressure sensor there is no change in idle.

FSM says to hook up a pressure tester to 100mmHG, turn ignition switch and check voltage. Can I simply blow in the pressure sensor and read the voltmeter?

TPS testing:
Narrow: .8 as I go to WOT it will read once at 1.06 then go into OL and then pick up and have one reading of 6.4 where it will hold

Full: .661 closed and as it goes to WOT it will range up to 3.8, then go into OL and pick back up 4.43 and hold.

[Rotary Evolution]

the compressor outlet doesn't show vacuum, only boost positive pressure and not the same pressure at the manifold leading into the engine so it isn't a good spot for the pressure sensor line. the sensor needs to see engine vacuum and pressure that the engine sees to accurately advance timing.

move the line to either the top or bottom front port just past the throttle body off those 3. the center port iirc goes to a port before the throttle body so isn't a good MAP source.

i highly doubt this is your problem but it is a problem that should still be addressed.

[JustJeff]

Originally Posted by Rotary Evolution

the compressor outlet doesn't show vacuum, only boost positive pressure and not the same pressure at the manifold leading into the engine so it isn't a good spot for the pressure sensor line. the sensor needs to see engine vacuum and pressure that the engine sees to accurately advance timing.

move the line to either the top or bottom front port just past the throttle body off those 3. the center port iirc goes to a port before the throttle body so isn't a good MAP source.

i highly doubt this is your problem but it is a problem that should still be addressed.

Yeah, I was thinking about just that while diving home for dinner. I need to get more vacuum hose to make any changes so that will have to wait till tomorrow.

Whats your opinion of using the same nipple that's under the BAC for Pressure Sensor, BOV, and aftermarket boost gauge?

Also I tested my father's S5 N/A TPS resistances out of a growing curiousity... and found the same areas of deadspots on the narrow range where the multitester goes into OL and then picks back up for one single reading and holds. I seem to see this with every TPS I've tested.

[Rotary Evolution]

the lower ports work but the closer the port is to the intake port in the engine the more resonance the signal will get. you can get by in most cases running an unfiltered line to the throttle body ports but the ones off the LIM should have an orifice in each line to each device otherwise they will 'flutter' in the readings.

i dislike teeing off more critical components like the map and pressure sensors. boost gauge and blowoff valve are less significant. albeit less significant to tee off the pressure sensor on a AFM/MAF based system, it is still used for engine timing calculations, so if a line off that tee ever does break it will advance timing to the ambient reading table which could lead to engine damage although i have never had it happen.

[RETed]

Originally Posted by JustJeff

Whats your opinion of using the same nipple that's under the BAC for Pressure Sensor, BOV, and aftermarket boost gauge?

#1 priority is the BOV valve.
Always use a dedicated vacuum line for the BOV.
Do NOT "T" this line, if possible.
If the BOV doesn't get a strong enough vacuum to trigger, it fails to open efficiently.
If the BOV doesn't open properly, it'll kill the turbo due to compressor surge.
I tend to use the fitting under the BAC valve for the BOV, since we're only concerned with vacuum.
The BOV opens due to a vacuum signal - i.e. when you lift off the throttle under high RPM's.
Pulsations under other conditions doesn't mean anything to the BOV; it stays closed under anything close to "0" or positive boost pressure.
This fitting also is usually the closest to the BOV in most installs (and with the stock unit), so having a short vacuum hose helps here.

Everything else doesn't really matter.
Unless you're worried about being off a few psi, they are not important.
Stock ECU retards ignition timing under positive boost - it does not affect fuel delivery.
The stock ECU ignition maps are very conservative, does being off a few degrees won't hurt anything.
Most aftermarket boost gauges aren't accurate enough for you to worry about being off a few degrees; even if the gauge is off, it's just a display.

Killing your turbo is a different matter...


-Ted

[JustJeff]

I'll keep all of this under advisement

What about using one of the fittings on the back of the TB for my boost gauge. I'm trying to remember what is back there. Large one feeds my OMP injectors via the splitter. I can't remember where I routed the other two, but I'm going to need to take the UIM off and look over my vacuum routing anyway.

Also where can I buy those orifices/pills for the vacuum lines (sorry I'm still working caffeine into my system so my thinker isn't too good ATM).

[RETed]

No problem using the fittings off the back (or the front) of the UIM.
I don't remember which one was which, but on the front of the UIM, the top and bottom fittings see vacuum - the middle does not.
In fact, if using a boost sensor or boost gauge, it's better to use those, cause they take vacuum off the surge tank; there is less chance of pulsations from the alternating front and rear rotors.

I dunno if Mazda still carries those pills, but I didn't bother with them.
The pill is to keep spikes from getting to the boost sensor, so the stock ECU doesn't trigger the over-boost fuel-cut unnecessarily.
If you have an FCD (or some way to defeat the over-boost fuel-cut), then this is not really necessary.


-Ted

[Rotary Evolution]

i don't recall any of the rear ports having vacuum, i use them for injector air bleed ports and the oil injector air feed port. the front upper and lower and the ones on the passenger side of the LIM split/ACV port are the only ones i use for vacuum+boost.

the orifice you may be able to find in the help section of an auto parts store. it just looks like a vacuum tee with a brass insert in one end(small hole drilled through), your original pressure sensor hose may still have an orifice in it, it is located about 2" upstream from the sensor in the vacuum hose. a vacuum line filter also works but they are a little more difficult to find.

[Pete_89T2]

Originally Posted by RETed

I dunno if Mazda still carries those pills, but I didn't bother with them.

The pill is to keep spikes from getting to the boost sensor, so the stock ECU doesn't trigger the over-boost fuel-cut unnecessarily.
If you have an FCD (or some way to defeat the over-boost fuel-cut), then this is not really necessary.


-Ted

I disagree with removing the restrictor pill from the MAP sensor line. If you do, a side effect is it will cause the MAP sensor to produce a "noisy" signal even at idle, which can effect your ability to tune the car. I have an Rtek 2.x modified N370 stock ECU, and when I was having Dave B. at Speed1 tune it for me, he initially had problems trying to determine my baseline on the 1st few dyno pulls. He noticed the MAP figures showing up in the Rtek logs were all over the place. Then Dave figured out I was missing the restrictor pill, popped a spare one he had into the MAP sensor line and all was good from there - MAP readings in the log were no longer noisy, and he was able to get a consistent tune from that point on.

BTW, you can usually find generic restrictor pills for 1/8" hose at Advance auto parts or Autozone.

[RETed]

It's odd that it'll affect the RTEK...
The stock ECU uses on open-loop process for idle, so I would think it would ignore the boost sensor signal.

I've gutted the stock ECU a long time ago and have been running a Haltech E8 for years now.


-Ted

[Rotary Evolution]

it's less of a problem on stock cars, the Rtek puts more dependence of tuning on the pressure sensor for the lookup tables for load, the stock ECU mainly uses it solely for timing advance.

i have also seen similar results on many cars without a restrictor in the line, even running the pressure sensor as far up stream as possible you sometimes will get resonant spikes in the readings. the Rtek was actually showing a bounce of between 5"Hg and 2psi under WOT full load off the stock port by the ACV on a non turbo car. the readings smoothed out from a 7"Hg bounce to 3-4"Hg bounce by moving it to the throttle body, installing the factory restrictor lowered it to less than 1"Hg fluctuation.

on some cars even with a standalone on turbo engines i have seen 2-3psi fluctuations even locating the source line at the port i mentioned near the throttle body, i always recommend an orifice located near your map sensor to filter the signal. i suspect this also may be due to some lax filtering of the signal on the ECU side of some manufacturers. those were cars that had either no boost controller or manual boost controllers equipped, wastegate cycling was not the cause.. the result was AFRs bouncing from perfect to 1-1.5 points lean.

[JustJeff]

I love all the input!! Feed me Seymour!! Keep all this knowledge coming!!

Here's what I've found thus far. I change the source for my pressure sensor to the UIM front top port and that did make a significant difference. But I still had a low RPM at idle and if I fluttered the RPM it would still hesitate and bog down.

Throttle cable was part of the problem. It had a lot of slack on it at the back of the TB. I tightened that up and that combined with changing the source for the pressure sensor has made a world of difference. It lops a little bit at idle but idles between 500-900ish idle. I'd raise RPMs up to 2-3k and let them drop back down and idle point would be a little different each time.

It still does a very small hesitation when I first flutter the RPM. Associated with that little hesitation I can hear something out of the ordinary coming from the engine. Sounds like something opening or closing, possibly a vac/pressure source...or maybe the throttle butterflies. It is always associated with the hesitation and only happens if I flutter the throttle. I was working alone so I couldn't stick my head in the engine compartment while someone else fluttered the throttle, but tomorrow I can get a helper.

I did not check timing today because I could not remember which timing marks were which and didn't have a spare main pulley to compare. I had mine powdercoated and then repainted the marks myself. IIRC first one is red/trailing, second one is yellow/leading?

One other thing very strange which may or may not be related to the problems. I have a huge amount of exhaust spray of soot and moisture. It was doing this after the initial rebuild and is still doing it. When I play with the RPMs and get them bouncing up and down, not higher than say 3k I have about a 4-5 spray.



I didn't get a close enough pic, but it's moisture with soot. I tasted the moisture and it's nothing but water...no coolant. Where is the moisture coming from? Is all of that from condensation in the exhaust?