HOW TO DO AN ADVANCED SEARCH.

Mon Jul 08, 2013 3:56 pm by Admin


For the benefit of 944Hybrids users there are two search functions available for you to use.
The purpose of this sticky is to explain the "Advanced Search" function because it is much more powerful and is the best choice when researching information.

When you log on to the site a list of options is shown in a line at the top of the page. One option is labelled "Search", use this option (NOT the search box lower down on the right).

After you click on the upper search option, a drop down box appears. At the bottom of this box is a radio button marked "Advanced …

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944 with LS3 bucking or jerking while cruising or low speed problem

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944 with LS3 bucking or jerking while cruising or low speed problem

Post  Porsche951toLS3 on Sat Jul 14, 2018 3:31 am

I do not see this problem in the forums so I need help please.

1986 Porsche 944 Turbo with new crate LS3 motor with the Hot Cam 480hp.

I just got it out on the road and I have only one problem that I cannot figure out.
While driving at low speeds or while cruising and no acceleration, the car is lightly bucking or jerking
back and forth. I feel like I need to accelerate just to make it stop. I have the Drive by Wire pedal.
I do not have any engine light or error codes.

My idea was to brace the transmission like toyota trucks do with there rear end to stop this, but I am
wondering if anyone else has had this problem. I don't know if this is mechanical or tuning.

I have gone through the whole car and the suspension is all new......very frustrated and I cannot find any mention of this in the forums.

I appreciate any feedback.
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Porsche951toLS3

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Re: 944 with LS3 bucking or jerking while cruising or low speed problem

Post  Arthropraxis on Sat Jul 14, 2018 9:24 am

My car had some surging at low speed constant throttle until it was dyno tuned. I also run VSS, some people do not, which is supposed to help smoothness at low speed.
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Re: 944 with LS3 bucking or jerking while cruising or low speed problem

Post  sharkey on Sat Jul 14, 2018 12:10 pm

bracing the transaxle will only make the bucking worse. its not mechanical, its electronic. like mentioned above, if you dont have the vss kit in the car, put one in, the ecm needs it. past that, you need tuning.

sharkey

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944 with LS3 bucking or jerking while cruising or low speed problem

Post  Porsche951toLS3 on Sat Jul 14, 2018 12:38 pm

Idea
Your feedback is saving me from doing stupid things.


Last edited by Porsche951toLS3 on Sat Jul 14, 2018 12:52 pm; edited 2 times in total
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Porsche951toLS3

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944 with LS3 bucking or jerking while cruising or low speed problem

Post  Porsche951toLS3 on Sat Jul 14, 2018 12:48 pm

I have a VSS sensor and 40 tooth wheel installed.  Since I do not know if this is correct I am throwing out my gap which was set at .80mm.  I did not have instructions for gap, but I found this setting while reading online.  Is my gap ok or should it be closer.  Additionally I used 16 AWG silicone wire to extend the sensor all the way to the back. Is my wire too large and is my gap too wide?

Maybe the question before going to the tuner is how do I verify the VSS sensor is working?  I can look at the app "Torque" using my phone and an ODBII dongle.  Would seeing the speed of the car come from that sensor?  

I have also noticed that sometimes when coming to a stop it will dip down below idle and then recover but one time it stopped the engine.  As I think about your comments, these all sound like the VSS sensor issue.  Additionally I have my MAF in an Airraid tube in the nose of the car well over 10" from the intake.
I don't know what information someone may ask for.  

I guess I need to do more homework, but ANY FEEDBACK is APPRECIATED.  This is my only little frustrating issue.  Wisdom from others can stop all my endless stupid trials and errors.  

Thank you for the comments so far.  You stopped me from making braces to fix this.


Last edited by Porsche951toLS3 on Sat Jul 14, 2018 2:08 pm; edited 1 time in total
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Porsche951toLS3

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Re: 944 with LS3 bucking or jerking while cruising or low speed problem

Post  sharkey on Sat Jul 14, 2018 2:05 pm

yea you can use the torque app to monitor the vss signal, see if its working and see if it is anywhere close to actual speed. with the idle dropping and stalling during coast down, it sounds like a vss related issue.

the maf doesnt care where it is in the intake tube, just as long as its in a straight piece of tubing thats the specs from the manual. ive dealt with a few gm crate engine installs and even when the car is put together with everything built to the letter of the manual it still needs tuning. what i find in gm vehicles is anytime you do anything to change the airflow in the air intake (be it a cold air intake, or a crate engine that doesnt use the exact same intake the tune was written for) you will need to calibrate the maf tables to make it right. you will be able to see how far out things are in the fuel trims, your ideal long term fuel trims (ltft bank 1 and 2) should be + or - 4% in a perfect world.

id recommend having a look at things with the torque app, make sure the vss is working and go from there.

sharkey

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944 with LS3 bucking or jerking while cruising or low speed problem

Post  Porsche951toLS3 on Sat Jul 14, 2018 2:53 pm

NEW INFO - My Torque app reads the VSS sensor at approx 13mph when I am doing 50mph.  Sounds like my problem.

So now how to fix it please. I will be doing a lot of homework to understand this problem. 

1.  My VSS to ring gap is set to .80mm which may be too wide
2.  I used 16AWG wire to extend the VSS sensor plug......is that too large of wire?

ANY OTHER SUGGESTIONS or do I have a bad sensor?  I am open to everything and excited to
have the possible answer thanks to you guys who answer this stuff.  I am very appreciative.
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Porsche951toLS3

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Re: 944 with LS3 bucking or jerking while cruising or low speed problem

Post  sharkey on Sat Jul 14, 2018 4:04 pm

im not sure if you can look at data on a graph with the torque app, but if you can look at the graph and see if its a smooth signal. if the graph is smooth id have no reason to suspect the gap is too large. in fact, id suspect the gap may be too tight, .8mm seems a little on the tight side, however that can depend on the tooth depth on the reluctor wheel as well. again, if the signal is smooth id say its just fine. same deal with the wire size, it shouldnt affect it, but if it did you would see a jumpy signal.

my thoughts are its in the calibration. im not sure what they set the crate engine ecm with for a vss trigger wheel.

sharkey

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Re: 944 with LS3 bucking or jerking while cruising or low speed problem

Post  sharkey on Sat Jul 14, 2018 4:21 pm

it just dawned on me i have an e67 gm crate engine base calibration from a car we built at our shop. i had a look and it shows the vss and it does indeed show a 40 tooth trigger wheel, however it also has a gear ratio of 3.91. the difference here is the calibration is assuming the speed sensor is on the transmission output shaft in a normal vehicle, essentially between the trans and differential. our vss setup is after the final drive, and is at wheel speed, not driveshaft speed. the ecm is using the rear end ratio setting as a divisor for the speedo. if you work backwards,

50 mph / 3.91 (the gear ratio setting in the ecm) = 12.8 mph, round it to 13mph.

the answer, your going to have to find someone to go into the ecm with HP Tuners and change the setting. its as simple as changing the gear ratio box from 3.91 to 1 and flash the calibration in.

sharkey

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944 with LS3 bucking or jerking while cruising or low speed problem

Post  Porsche951toLS3 on Sat Jul 14, 2018 10:08 pm

I think you hit the nail on the head. While you figured this out, I went out to the car and:

1. Double checked my gap - no problem.
2. Ran the car using "Torque" in graph mode - No Noise
3. Everything reads approximately 25% of my actual speed
4. Your calculations are dead on to my problem.

Thank you............I have my answer to get a tuner to set my VSS correctly.
I will write back on this forum after I am done to confirm results.

**YOU HAVE FIXED MY PROBLEM. I AM HUMBLED AND APPRECIATIVE FOR YOUR GENIUS***
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Porsche951toLS3

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Re: 944 with LS3 bucking or jerking while cruising or low speed problem

Post  sharkey on Sun Jul 15, 2018 1:14 am

im just glad i can be of some help.

sharkey

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Re: 944 with LS3 bucking or jerking while cruising or low speed problem

Post  superman22x Yesterday at 9:24 pm

It does not matter if you are reading 13mph or 50mph, the software only really cares if you are in neutral or drive, and with the manual transmission it estimates this by looking at vehicle speed. And even still, if you are on the pedal at all, it doesn't matter. It's really only for coast mode.  

Do you have a 4" intake?  I also run an LS3 and had the same problem.  I had originally ran a 3" intake, and had the problem before I changed a cal around the MAF.  There is a cal that converts frequency seen by the MAF to airflow.  The card style MAF takes a tiny sample of air and assumes you are flowing the same amount of air through a 4" diameter pipe.  I'm not sure what is available in HPTuners or EFILive for this, but I imagine there is something along these lines.  This is all for a Gen IV controller.

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Re: 944 with LS3 bucking or jerking while cruising or low speed problem

Post  sharkey Yesterday at 10:34 pm

looking at the ls3 crate tune i have, the ecm doesnt increase the "idle speed" till 8mph. with the gear ratio settings in the ecm that 8 mph is 31 mph actual speed. this may not be the whole issue, but its for sure a problem.

the air intake is for sure another issue, and this is why i recommend tuning. like i said above, the tune for the crate engine is based on a specific air intake design and any deviation from that design will cause the tune to be out. same with headers and exhaust design, it wont change things as much as the intake side, but it needs to be adjusted for. it would be like slapping a holley carb on a small block chevy crate engine and expecting it to be 100%, its just not gonna happen. ive yet to hear of an ls crate engine that was tuned and didnt see gains in power and driveability.

either find a local tuner thats familiar with the gen 4 ls stuff, or take this as an opportunity to purchase hp tuners and learn how a modern efi control system works. the gen 4 controllers are very advanced and complex to tune, and it takes hours upon hours of reading and learning to get into. however all you need to do right now is sort out the vss settings, and recalibrating the maf vs frequency tables, and thats not all that difficult as far as things go. being able to tune your own car is very rewarding.

sharkey

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Re: 944 with LS3 bucking or jerking while cruising or low speed problem

Post  superman22x Yesterday at 11:51 pm

I assume you are cruising above idle speed. Idle speed will only matter when in speed control when you are coasting, unless there are some really high values in that table (like if you have an engine cal for an auto trans).

The gen IV stuff is complex, it's very model based. A lot of tuners don't understand it well really, even the ones that say they do.

superman22x

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944 with LS3 bucking or jerking while cruising or low speed problem

Post  Porsche951toLS3 Today at 12:44 am

I contacted three tuners and picked one who actually addressed the issues you all have stated in these forums. I am now fully understanding why I definitely need this tune. I have the 4" intake all the way with no restriction, but the MAF is in the nose so much further away from it's intended location. I also have the headers from Renegade Hybrids which I am sure changes things as well as my incorrect VSS sensor ratio that sharkey figured out.

I now fully understand with everyone's help why this tune will fix my problems. The person I picked just tuned a Porsche 944 with the 525hp LS3 for a guy across town so he is already familiar with what I need and he has 12 years of tuning experience.

He comes over next Monday. I will update this forum for future readers on this topic. I hope to have a great write up on my results.

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Re: 944 with LS3 bucking or jerking while cruising or low speed problem

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