Hello, I recently swapped out the L98 in my 91 Formula for an L31 with the TPI intake adapter plate after my original engine developed a crack in the block. The new engine runs great on the highway but has a persistent idle problem at a stop, in park, or anytime there is not throttle input. A poster in another thread let me know that running the original L98 prom with the L31 Vortec head engine is likely the problem. Can anyone here help me learn where to begin with getting a new prom tuned for the L31 that will hopefully correct this problem? Thank you very much for any help that you might be able to provide.

 

Many people have done this same swap and used the stock PROM and had no issues. Compared to the L98, the L31 has better flowing heads, but a little less aggressive cam and in the end it's pretty much a wash.

Your issue sounds like it could be a vacuum leak (common at the intake runner gaskets). Did you also set the ignition timing, fuel pressure, IAC and TPS to factory specs?

 

Too bad You didn’t put the L98 cam in the new engine. The L31 Cam is smaller than the L98 cam.

 

Thank you for your responses, I'm trying to get a handle on this issue and have a lot to learn. Do you agree at all with the below that was posted in another thread about my problem?

"If you are trying to run a vortec head engine on a tpi chip, this is the problem. The vortec head only needs a maximum of 32 degrees timing vs. 38 for a tpi.
The reason it idles better with the est unhooked is the ecm isn't able to change the timing. It needs a tune."

 

I agree that less timing is needed for the Vortec heads, but i don’t believe that will cause the idling problem you are describing. It would more likely show up as detonation under load, you could try set the base timing at the distributor from the stock 6 degrees btdc to 0 btdc to try out this theory.

 

Set the base timing @ 6* unhooked EST. After setting the timing, turn the car off. Plug up the EST wire then you are don with the timing. Adjust the fuel pressure regulator if you have one to 42-45psi. Any other timing that needs to be done is through the ECM(burn a chip). Your TPS may be off as well.

 

Thank you all, I will start out by setting the timing back just in case that helps. Will let you know what is happening!

 

Update - it looks like retarding the timing back to the votec levels did not solve the problem, the idling behavior of the car is essentially the same. I will go back to looking into PROM tuning as the possible solution along with making sure there are not any vacuum leaks. Thank you for any other ideas that you might have!

 

Clean the throttle body and iac passages. It may also help if not already done. Definitely check for vacuum leaks

 

Question, are you running the external egr adapter kit for vortec? Since vortec heads do not have internal egr passages, you use an adapter kit to run egr externally. The problem you are describing is what will happen when you are getting egr at idle. EGR is suppose to only come into play at speeds above idle. Of course if you deleted egr than this will not be the problem. Good Luck.

 

Hello, no we are not running an EGR adapter and I know that after a while the computer will throw the EGR code. Can you tell me more about this and the adapter since I am new to this? We did not delete EGR from the prom or anything like that, my understanding was that EGR would simply not be hooked up but that it wouldn't cause a problem (which seems to not be the case as we are experiencing issues). Thanks so much!

 

Scoggin Dickey use to make a corvette style rear feed pipe for vortec conversions.
The error code 32 shouldn’t show with the solenoid intact even though the egr isn’t getting exhaust gasses.
Have you considered dynamic efi ebl p4. I believe rbob has made a bin that has l31 timing table.

 

Tuned performance is correct. Scoggin Dickey sells or used to sell the egr adapter kit for votec TPI intake manifolds. You drill a hole in the exhaust manifold or header tube and weld-in a bung. Attach metal tubing that runs to a port on the rear of the intake manifold. Since you are not running egr that rear port on the intake should have a plate over it. EGR cools the comubustion process, thereby reducing Nox and allowing more timing to be run without detonation. I doubt if not having egr deleted out of the ecm programming is causing your problem. If you don't need the egr to pass inspection I would recommend just tune it and delete egr from the programming.

Concerning the error code, TPI has a temp sensor located on the egr valve. I'm not sure if all years have this. When the ecm calls for egr, it monitors that temp sensor. If it does not detect a temp increase then the ecm assumes the egr valve is not functioning properly and sets an error code. I'm not sure if the error code causes the ecm to change how the motor runs.

Since your running speed density, dynamic efi ebl p4 would be a good idea if you willing to do your own programming.

 

I'm starting to think that it's possible that there are not EGR block off plates in the intake on my car, would that cause this type of idle surging/bogging issue? Looking back at all the steps on the L31 swap guide thread I'm thinking that the block off plates might be the one step that the shop did not take when doing the swap.

Again I have very little knowledge in this area - but it sounds like if you do not want to run EGR on an L31 swap you have to create the block off plates, and if you do want to run it you have to tap into the header with an adapter kit like what is being mentioned by FredSS and Tuned Performance is that right? I'm afraid my car doesn't have it blocked off and maybe that is causing all the problems I'm experiencing. I know for a fact it doesn't have the EGR adapter kit.

 

Yes, this is correct, but easy to check. There are two locations for block off plates. One is at the egr valve connection to the lower manifold. If the egr valve is in place, that’s fine, it will seal the hole. The other is for the external egr pipe connection. It’s at the passenger side of the lower intake manifold, between the #8 intake runner and the distributor.

You can see the two locations in the photo below.

[img]blob:https://www.thirdgen.org/1332f2c8-d82b-455d-a8b8-fcda3c549f0f[/img]

 

Thank you very much this is a big help!

 

https://www.ebay.com/itm/Dynamic-Efi...EAAOSwVVFdM0C6

 

TransamGTA350 can you repost the link to that photo? The link doesn't seem to be working. Thanks again!

 

 

Perfect thank you!