Jump to content
sandroosten

Ls1 Backfire & Limp Mode At Wot Above 5k Rpm

Recommended Posts

I need some help from some of the professionals on here. I have a Tatum black widow with a LS1 and a MEFI that is supposedly tuned for 500HP. (that’s what it says on the bottom of the computer). My issue is that when I am at wide open throttle or close to it and the engine goes over 5000 rpms it backfires and goes into limp mode or power reduction mode. I have to shut the car off and turn it back on and then it runs fine. I have beat my head against the wall trying to figure this thing out. The car had this issue when I bought it, so it is nothing that I did that may have caused the issue. If I ease into throttle and slowly increase the rpms it will go up to about 6000 rpm but the second I give it a quick push to the pedal it backfires or misfires and I have to shut it down.

I have fuel pressure even when under a load (I have not checked the injectors )

It has brand new spark plugs in it.

Brand new fuel filter

Many people are telling me it could be a tuning issue. The computer is from turn key

I purchased the mefi scan tool to try and figure it out but its not throwing any codes.

Any help is appreciated.

Share this post


Link to post
Share on other sites

I'm guessing voltage drop at WOT, I had this problem years ago.

I cut the hot wire from the ignition switch to the computer and installed a relay going straight to the battery.

I talked to the guys at Turn Key because my motor came from there.

This is what they told me to do and it worked like a charm. :sraptor:

Share this post


Link to post
Share on other sites

thank you for the reply. any info on the specific relay you used?

Share this post


Link to post
Share on other sites

I just used a 30 amp auxiliary relay you can buy at any auto parts store.

The amps are way overkill but it works great.

Share this post


Link to post
Share on other sites

Look it up on Amazon its like $4.00 with the plug.

post-54050-1459783573.jpg

Share this post


Link to post
Share on other sites

We had a similar issue, it was the crank sensor. There are two types, a gray or a black one. Match the one you have.

Good luck.

post-6971-1459784119_thumb.png

Share this post


Link to post
Share on other sites

Do a sweep test on the TPS and see if it drops out.

Share this post


Link to post
Share on other sites

I really appreciate all of your feedback. I ordered the relays last night and it looks like I should order the crank sensor. Hopefully I can take this thing out this weekend and figure it out.

I do have a question about the low voltage at WOT. Im guessing this thing did not roll of the showroom floor with this issue and neither did Jaybirds. What would be the root cause of the low voltage at WOT? regardless Im going to do the relay and I am very confident that one of your suggestions will do the trick.

Share this post


Link to post
Share on other sites

adding a relay to the switched power should take care of it. a lot of older cars had this problem (including mine). the computer needs good clean power.

Share this post


Link to post
Share on other sites
I really appreciate all of your feedback. I ordered the relays last night and it looks like I should order the crank sensor. Hopefully I can take this thing out this weekend and figure it out.

I do have a question about the low voltage at WOT. Im guessing this thing did not roll of the showroom floor with this issue and neither did Jaybirds. What would be the root cause of the low voltage at WOT? regardless Im going to do the relay and I am very confident that one of your suggestions will do the trick.

People tend to add electrical devices and components to their cars,

and even if they hook them up correctly it can draw down on the voltage that is going back to the computer.

In my case I upped the size of my fan and even though the fan is on a relay the computer was not getting enough voltage.

Once I installed the relay for the computer I was home free. :sraptor:

Share this post


Link to post
Share on other sites

ok,

I got the relay in the mail yesterday along with the crankshaft positioning sensor. Getting ready to start cutting. I just want to confirm with you guys who are much smarter than me that I am doing this correctly. For the relay I have the Mefi 4 and the orange wire in the #1 spot is a constant hot that I should not mess with. This wire is always hot regardless of the switch. # 19 (the pink wire) is the ignition feed that is only hot when the switch is on. It is # 19 that I am going to cut and put the relay in with a direct feed from the battery right?

I am going to test the relay first, and if that does not fix the problem I’ll change out the crankshaft sensor. I don’t want to do both at one time so that when I do fix the problem I know which item was the issue. Again all of you have been so helpful and it is much appreciated.

post-94813-1460286187_thumb.jpg

Share this post


Link to post
Share on other sites

well, I went and tested today. I thought I had it. seemed to be running very good, but once the motor warmed all the way up it was right back to the same old thing.

i ran the direct wire from the battery with the relay to the ignition feed.

weird thing was when I put in the new crankshaft positioning sensor the car would not start at all. put the old one back in and it fired right up.

Share this post


Link to post
Share on other sites

Looks to me like you did it the right way.

Hopefully someone else will chime in with some more ideas.

Sorry the relay didn't help you out.

Share this post


Link to post
Share on other sites

Is it a speed density or mass air flow.

I think you said a Mefi setup so that means speed density. So your car runs good until

It goes in to closed loop. Then other sensors like the map sensor starts trimming the fuel.

So I'd try to bench test the map sensor.

Share this post


Link to post
Share on other sites

Jaybird95- No worries. I really appreciate the reply's and the the help. It was a inexpensive test!

Sandtours - Ill give that a shot. I need to go buy that vacuum pump.

rampage42 - I did attempt to change the crank sensor. I bought a cheap one off ebay and it appears it was DOA :nag: . Ill go to the dealership and buy an OEM one. I have been told that the CPS is either good or bad, and bad wont run at all. But I'm willing to try anything to get this stupid problem behind me. chasing electrical sucks.

I have the MEFI scan tool. I only purchase the the base model to read what was going on so I am unable to do any tuning, and for some reason it is not recording the data. Is there anything I should be looking for that could be a give away?

as far as codes its not throwing any, but it does show a "check engine output" a "closed throttle" and a "stall or reset". Are these normal items to show or is the computer trying to tell me something?

post-94813-1460436280_thumb.jpg

Share this post


Link to post
Share on other sites

what will a car do if the tune is for 100 octane and I have been running 91? will it show the symptoms I had mentioned above?

I know its a stupid question and it is probably something I should have known.

the previous owner didn't tell me and with it not having a turbo or a supercharger I didn't think much of it.

while I was out messing with it last night I had the laptop hooked up to it and I seen an option to view "octane" when I clicked on it is says 99.6 which I'm assuming is 100 octane.

obviously the answer is to drain the fuel and add 100 octane and retest. I am just stuck at work and its all I can think about so I am hoping someone might know if this could have been the issue to put my mind at ease a little. I wont be able to get back out to test for a couple of days.

Edited by sandroosten

Share this post


Link to post
Share on other sites

Not burn all the fuel efficiently. And it will run worse than with 91.

Wait... I read that wrong. It's tuned for 100 and you run 91.... Well that will lead to detonation. And I'm not sure how your tune works but if when you are wide open and it adds timing that could be bad.

Edited by jtmoney714

Share this post


Link to post
Share on other sites
Not burn all the fuel efficiently. And it will run worse than with 91.

Wait... I read that wrong. It's tuned for 100 and you run 91.... Well that will lead to detonation. And I'm not sure how your tune works but if when you are wide open and it adds timing that could be bad.

when you say not burn efficiently and run worse than with 91 what would be the symptoms? just lack of HP or would it include the stuff that I am complaining about?

Share this post


Link to post
Share on other sites
Not burn all the fuel efficiently. And it will run worse than with 91.

Wait... I read that wrong. It's tuned for 100 and you run 91.... Well that will lead to detonation. And I'm not sure how your tune works but if when you are wide open and it adds timing that could be bad.

when you say not burn efficiently and run worse than with 91 what would be the symptoms? just lack of HP or would it include the stuff that I am complaining about?

Disregard that sentence. When I first read it I thought you said it was tuned for 91 but you run 100.

Share this post


Link to post
Share on other sites
Not burn all the fuel efficiently. And it will run worse than with 91.

Wait... I read that wrong. It's tuned for 100 and you run 91.... Well that will lead to detonation. And I'm not sure how your tune works but if when you are wide open and it adds timing that could be bad.

when you say not burn efficiently and run worse than with 91 what would be the symptoms? just lack of HP or would it include the stuff that I am complaining about?

Disregard that sentence. When I first read it I thought you said it was tuned for 91 but you run 100.

ok, but same question. would this cause the symptoms that I am experiencing?

Share this post


Link to post
Share on other sites

IF YOU CHANGED CRANK SENSOR AND NO START COULD HAVE BROKEN WIRE IN THE CIRCUIT INSIDE INSULATION MAYBE GOOD AREA TO CHECK AND

POSIBLE CHECK END PLAY FOR CRANK SHAFT

Share this post


Link to post
Share on other sites
IF YOU CHANGED CRANK SENSOR AND NO START COULD HAVE BROKEN WIRE IN THE CIRCUIT INSIDE INSULATION MAYBE GOOD AREA TO CHECK AND

POSIBLE CHECK END PLAY FOR CRANK SHAFT

would that still be the issue if I put the old one back in and it fired right up?

Share this post


Link to post
Share on other sites
Not burn all the fuel efficiently. And it will run worse than with 91.

Wait... I read that wrong. It's tuned for 100 and you run 91.... Well that will lead to detonation. And I'm not sure how your tune works but if when you are wide open and it adds timing that could be bad.

when you say not burn efficiently and run worse than with 91 what would be the symptoms? just lack of HP or would it include the stuff that I am complaining about?

Disregard that sentence. When I first read it I thought you said it was tuned for 91 but you run 100.

ok, but same question. would this cause the symptoms that I am experiencing?

Honestly I wouldn't know. I'd imagine you would hear some pinging. Would the ecu shut its self off to protect the motor?

Share this post


Link to post
Share on other sites

Couple of things to check

Voltage under a load out of alternator

Remove crank sensor look in the hole and see if the reluctor wheel looks straight. Black crank sensor (24X) can only have about .015 of run out

Spark plug gap should be .030 or so

Fuel pressure under load

Share this post


Link to post
Share on other sites

Maybe TPS (throttle position sensor) is bad at full throttle.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Preview Post

More Links

©2001 GlamisDunes.com.
All rights reserved.

×
×
  • Create New...