tillyvick14

Hello, I recently (with the help of my mechanic since I'm not too knowledgeable myself) swapped an L31 into my 1991 Formula after the original L98 developed a crack in the engine block. The TPI system was reused and we installed new injectors, everything else is stock and the throttle position sensor and idle air controller are new. We have never been able to get the car to idle properly with the new engine and have done the idle re-learn procedure many times. The car runs beautifully on the highway but after it is allowed to warm up it will bog down and drop about 300 rpm when idling at a stop and will often die. The idle problem only appears after it has been driven for about 30 minutes and fully warms up. The car also does not idle properly in park and surges up in rpm while in park instead of the bogging down that it does while in drive after being warmed up. We just replaced the ECM after my mechanic noticed that the problem was cleared up somewhat when the EST bypass was disconnected (this lead us to think that it might be a problem with the ECM) but the problem isn't any better with a new computer.

I would be grateful for any ideas that you might have to help me chase the problem. I have read many other threads here that seem to identify lots of potential causes for this. I'm wondering about the fact that I'm not able to run EGR due to the L31 swap and if that might be causing trouble for the computer. I'm also wondering if a new throttle body might help somehow. Thank you very much for any new leads that we might be able to chase.

aliceempire

iTrader: (14)

Idle surging tends to be a common issue with tpi. I'd look for small vacuum leaks. No promises though as vacuum leaks can raise the idle speed too.

sofakingdom

iTrader: (1)

EGR is probably not the problem.

More likely, it's related to this.



Unless of course you bought the $$$base$$$ for Vortec heads; or, used the original L98 heads on the later-model short block.

I agree that your problem is more likely to be a vacuum leak.

Details about the parts you used would help.

tillyvick14

Thank you very much for the replies. We did install this SD Vortec intake baseplate when doing the engine swap:

https://sdparts.com/i-23896057-sdpc-...SABEgLsMfD_BwE

I will ask my mechanic about checking for vacuum leaks again, I think he is confident that there aren't any external leaks that can be seen when checking with carb cleaner. Could there be something inside the throttle body as well? I've wondered if getting a new aftermarket throttle body would be a good idea anyway but it seems like some posters here have had bad experiences with them. Thanks again for your help with this!

aliceempire

iTrader: (14)

The problem with TPI is there are so many hard to reach places that could leak. A smoke machine or a DIY (cheap cigar, small paint can, fittings and tube) smoke machine will really help find a leak if you have one.

TylerSteez

iTrader: (3)

When I had TPI I would buy a black and mild cigar from the gas station, tape off the throttle body, remove a vacuum line, take a drag of the cigar and blow the smoke in the vac line,

This method found me plenty of obscure leaks.

tillyvick14

Thank you I will ask my mechanic to go the smoke machine route to try to find more leaks. Basically any time the car does not have any throttle input (at a stop idling, in park, or even when in drive when I'm going down a hill with my foot off the throttle) it will either surge or try to bog down and die and can't keep a steady idle. Does this sound most likely to be vacuum leak type of behavior?

aliceempire

iTrader: (14)

Yes, a vacuum leak can be a cause of those situations. Most are misdiagnosed due to tpi being a complicated intake.
Did you have the ecm/prom retuned for the new engine? If not then it really should be, especially if there's a decent amount of difference between old camshaft and new. Speed density systems like yours are more sensitive to vacuum changes from a cam change and leaks, though I'm still leaning to a leak.

BIRD91ZRAG

iTrader: (1)

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.

tillyvick14

Thank you very much, I am going to check into the timing difference now!

tillyvick14

I really want to keep pulling on this thread about the ecm/prom.... We definitely did not do anything to the prom and the timing is set at factory specs which would be for the TPI, so maybe that is our problem. Can you guys tell me how to tune the prom to be set properly for the L-31 engine with Vortec heads? Do I need to buy a new prom or can it be tuned/flashed somehow? Also would retarding the timing back to 32 deg work? Thank you very much BIRD91ZRAG and aliceempire.

BIRD91ZRAG

iTrader: (1)

You'll need someone who can burn a prom for your application. Timing is controlled by the ECM, there's pre-set tables with graphs, and each graph is different, and the ECM applies the correct table/graph based on the inputs from the TPS, MAP, MAF, CTS and other sensors.

You can read up about burning chips in that forum here. Good luck! I'm building an 82 that I bought an L31 crate engine for, but I'm going with a carb to eliminate the hassle of proms and tuning.