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Old 04-27-2013, 09:37 PM   #1
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Default 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.



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!
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Hesitation on acceleration-th_vid_20130422_191742_zps36de9bfe.jpg  

Last edited by Grantofhell; 04-29-2013 at 10:44 AM.
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Old 04-29-2013, 10:39 AM   #2
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Old 04-29-2013, 10:43 AM   #3
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Quote:
Originally Posted by Fathom55 View Post
Yes!

I checked mechanical timing yesterday, it was fine.
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Old 04-29-2013, 10:50 AM   #4
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TUNE THE CAR.
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Old 04-29-2013, 11:13 AM   #5
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Quote:
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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.
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Old 04-29-2013, 11:15 AM   #6
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Also, your AFR is going crazy when it stumbles... Can you post a log?
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Old 04-29-2013, 11:42 AM   #7
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Quote:
Originally Posted by Braineack View Post
TUNE THE CAR.
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 :\

Quote:
Originally Posted by Fathom55 View Post
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.

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.

Quote:
Originally Posted by Fathom55 View Post
Also, your AFR is going crazy when it stumbles... Can you post a log?
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.
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Old 04-29-2013, 11:47 AM   #8
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Quote:
Originally Posted by Grantofhell View Post
It has been tuned for many months. This came out of no where a few weeks ago.

I'll fix it up today and put logs and the tune up.
okay. good, this.
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Old 04-29-2013, 01:06 PM   #9
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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?
Attached Files
File Type: msq 2013-04-29_12.01.29.msq (81.1 KB, 78 views)

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Old 04-29-2013, 01:15 PM   #10
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does it slowly go up as well?
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Old 04-29-2013, 01:22 PM   #11
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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?
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Old 04-29-2013, 01:51 PM   #12
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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.
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Old 04-29-2013, 02:12 PM   #13
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I will most definitely share my findings. Just need to get my datalogs up. I'll try a 3rd party host.
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Old 04-29-2013, 02:21 PM   #14
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Attached Files
File Type: msl ErrorLog-4-29.msl (34.7 KB, 72 views)
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Old 05-07-2013, 12:04 AM   #15
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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.
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Old 05-12-2013, 01:38 AM   #16
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Last bump before the engine gets pulled. Any ideas?
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Old 05-19-2013, 06:38 PM   #17
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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.
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