Hesitation on acceleration
 

I'm not normally one to ask questions to the forum since I much prefer lurk/hunt for answers but I've been baffled now for about a week on this one. I've been searching using different tags but haven't come up with a solution to the acceleration problem, or the sticking throttle body.

I'll try to find a way to get datalogs as soon as possible. My tuning laptop is broken :(

Info:
'95 1.8 boosted running on MSExtra 2 so CEL checking isn't possible (not that I know of). Car has been running well up until now and no recent work has been done that would suggest cause for either problem.

Problems:
-Car hesitates/stumbles when throttle is applied. Sometimes dies when I try to come off a light because the RPMs drop so much.

Have already tried:
-Pulled TB and cleaned in and around (I know that wears the bore seal but I'm desperate here)
-Ran the car without boost/intake tract to insure intake blockage was not a problem.
-Fuel pump works, replaced fuel filter.
-Changed plugs (NGK BKR5E11-EGP) and the wires (Duralast) are relatively new and have been working fine for my application, despite the cheap-ness.
-Checked injector o-rings for leaks.
-Car achieves peak boost target (15psi) so vacuum leaks are not suspect. Still possible but hasn't been the case in my previous experience.
-Mechanical timing and spark advance tables.

Have not tried but will, time permitting:
-Checking fuel pressure

Below is a link to a video which shows the symptoms. Sorry about the poor quality.

https://www.miataturbo.net/attachmen...ine=1367109427

I'm open to any and all suggestions for possible cause, including ones that seem far-fetched. If any more information is needed, please do not hesitate to ask.

Thanks in advance!

Similar issue? https://www.miataturbo.net/megasquir...h-wrong-72463/

Yes!

I checked mechanical timing yesterday, it was fine.

TUNE THE CAR.

This. Or so my limited knowledge tells me...

How does the car react if you are driving down the street in 3rd gear at ~4k-5k rpms and let the car engine brake (no throttle), then give it gas. Does it still want to die or lurch?

I don't think mechanical timing will be the culprit.

Also, your AFR is going crazy when it stumbles... Can you post a log?

It has been tuned for many months. This came out of no where a few weeks ago. I hadn't made any changes in the tune and it ran very well. Idle was smooth, AFR's were on/very near target for each cell, etc. Even if the tune is the problem, I've no idea where to start :\ :noob:


Yes. It does that from any RPM and at any %TPS, whether I'm already on the throttle or not.

I didn't think so either, but it was worth a look.

I'll get a log as soon as I can. My laptop with which I was initially tuning broke and the repair process on it has been dragged out from long work hours. I'll fix it up today and put logs and the tune up.

okay. good, this.

i'm horribly ashamed of the tune, but here it is.
Data log of a short drive in the rain today is also attached. Some idling, revs, pulls, and downshifts

%TPS is quite interesting since it goes to well over 100 percent then slowly drops back down..Time for a new TPS or just a calibration?

does it slowly go up as well?

No, it shoots up to about %160 very quickly.

For some reason, I cannot upload the .MSL. It's under the 4.81MB filesize limit, but will not successfully upload. Any other media host you can think of that I can share this via?

If you figure it out, please post results. My car started doing the exact same thing before I took it apart. I'd love to know what your results are, I'm hoping mine is cured if I ever get it back on the road but I doubt it.

I will most definitely share my findings. Just need to get my datalogs up. I'll try a 3rd party host.

ErrorLog 4-29.msl - Speedy Share - upload your files here

Got it.

Just a bit of an update.

I tried replacing the coilpack(s) with known good unit(s) and no change. Changed TPS with a known good unit; no change. Pulled the fuel pressure regulator and "tested" it by applying 25mmHg vacuum and the diaphragm did not move. I don't exactly know how the FPR works but I assume the vacuum is modulated by the FPR solenoid which opens/closes the diaphragm to allow more or less flow of fuel return.

However, I did get some more insight (which made me think coilpacks):
It gets considerably worse when it's warm out and the engine is already hot. Applying throttle will stall the car after a while driving. Barely maintains idle either. Pulled the plugs and they were blackened. Tried new ones; no change.

The car goes off the road soon for the boost build. I'm on the fence about fixing it with the engine still in, in case the problem resides after I swap in the built block. As well as for educational purposes if this happens again or to someone else.

Last bump before the engine gets pulled. Any ideas?

As an update to this thread, during the process of my new build, I'm inspecting all sensors and mechanical bits to see what the problem was initially.

I found the mechanical timing to be off by two notches (teeth). I checked the mechanical timing while I was having the issue and it was 1 tooth off but I corrected the it. Must have been the idler or tensioner because it jumped again. I should have looked at this earlier because I had this exact problem with my mx-3 GS when I bought it in non-running condition.

I'll still continue to check parts and update findings.