I have a 1984 corvette with stock crossfire and no power mods. Recently did intake manifold gaskets and replaced everything from there up as the car was sitting for almost 7 years and everything dried up and started cracking. New gas tank, injectors, fuel lines etc. Starts and idles beautifully with no misses, base timing is set at 6 degrees.

The issues come when you rev it. A quick blip of the throttle makes the car stumble and backfire. Smooth application of the throttle and the cars revs smooth and strong. Checked timing as rpms were slowly increased to about 2k. At around 1500 rpm the timing immediately switches to somewhere around 15–20 degrees, almost off the timing marks.

It is obvious the computer has been messed with. A hyper tech prom was installed before I got the car. I also have a code 51 or 52 which I believe is related to the prom most likely because I took the prom out to inspect it and was not careful with it.

My question is, have any of you seen something like this with the timing curve and could this have to do with the prom.

 

The DTC could easily be related to the prom. If you could find a stock one to replace the aftermarket it one to test it. 51 is a substitution diagnostic check prom pins an installation replace prom but really it was ECM because proms weren't stocked. If ECM didn't correct it order and replace prom.
The other.sounds like a MAP sensor problem. Check fuel pressure 9-15 psi closer to 15 the better. T in vacuum gauge as close to sensor as possible. Vacuum gauge should move with throttle. A quick snap of throttle vacuum gauge should drop and recover instantly.. Carbon build in throttle body would be possible cause.

 

Thanks for the tips on the prom. I have a new one from hyper tech on the way (only brand new prom i could find). I spent hours cleaning the intake and replacing vacuum lines so I don’t think carbon would be an issue. I’ll try disconnecting the map sensor and seeing if there is any noticeable change. I believe my problems could all be caused by a bad prom however and the computer is just using a “limp” mode so to speak.

 

Also, interestingly enough, when I disconnect the est connector to do timing the check engine light turns off but the car still runs the same. Leading me to believe the ecm is fried as well. The vette did sit outside with clogged cowl drains for almost 7 years.

 

How does the car drive? The ignition curve does funny things when there's no load on the engine. It will be much different when you're actually driving the car.

 

Sadly the car is in the back of my barn and some other cars/ trailers i store for people are blocking it in. Gonna have to wait for spring unfortunately.

 

What is your fuel pressure? Back firing through the intake is a sign of too-lean air fuel ratio.

I'd also add more timing than 6* base. If it were ME, I'd start at 10 and expect to add more once I had it on the road where I could assess what it "wants", better.

I agree with Kevova about finding a stock PROM and putting that back in.

 

Fuel tank, sending unit, pump, lines, filter, regulator on both tbi units and injectors are all brand new and fuel spray is good so im not as concerned about fuel pressure. After reading up on the hypertech chip it does say that it changes the timing curve. So it very well could be that because the prom is fried the computer is reverting back to a timing curve that isnt for full power.

 

firing order ??

 

Cars idles smooth and quiet. Dont think it would be firing order

 

New parts ≠ enough pressure, necessarily. Spec is 9-13, and 9, 10...11 are definitely pretty stingy...the motor "likes" more than that. 12-13 minimum, IME.



I don't think the ECM has any "timing curves" in it....that is all programmed into the PROM.

 

A snap to 15 or 20 at 1500 rpm suggests that only the ICM is advancing timing. What is your base plus advance with the EST wire hooked up. It should be around 15 idle like that and advance to around 25 at 1500. The stock 84 timing curve ramps in pretty quick... but then again mine was a manual and the curves are different.

 

You're correct that the primary/normal spark curve is programmed into the PROM, but the distributor 7T EST module has a timing curve built into it to provide a fault-mode timing curve (for driveability purposes) in the event that the ECM takes a ****.

 

Got the new prom in today and it runs SUBSTANTIALLY better than it did before. Revs hard and fast no misses. The next thing now is the cooling fan doesn’t come on at all. Going to check wires and grounds and stuff tomorrow. Worst comes to worst I can just put a switch in the interior for the fan.

 

I presume you are aware that the prom has nothing to do with the cooling fan in the '84. It's a simple circuit . . . Coolant temp switch (CTS) in RH head, between plugs 6 & 8. Relay on LH fender panel, under m/c. Fan motor. That's IT! No connections to the ecm.

 

Hey man. Just going to tell you that if your ECU ends up being fried it may be worth your bank to just ditch the TBI's altogether. Go carb if your state emissions allow it or go to an aftermarket fuel injection system. If you ever start having problems with the units themselves it gets to be a nightmare real fast. take it from a guy who tried to make power out of an X-fire setup and sunk about a grand in for nothing but disappointment. (Power can be made from TBI's they are just incredibly temperamental and spending the money to get away from it now is going to be a lot better for you). Also consider a non-computer controlled ignition and going back to a conventional distributor or if your gonna do a computerized system get the full MSD package. (Worth it). Finally i just wanna clarify that I'm not an expert by any means and there are lots of more experienced, talented, and intelligent members on this forum than I who have made TBI work and built power out of those motors. And Kudos to them for it! I'm just trying to help you save money and a PILE of headaches in this instance. Cheers!

 

 

I don't know...mine seems to run pretty good with a few mods. When done correctly, they will run well IMO. Just because you changed all those parts doesn't mean that the FP is OK or where it needs to be. If you don't know where it is now, there is no way you can say the spray is OK from the injectors and the FP is fine without checking it. The FP can still be low and like mentioned, all CFI motors like the FP to be at 13psi minimum. On a stock motor, you really shouldn't have to go higher than that, but some people do, your choice. Are you still setting a CEL after the PROM replacement?

 

Here^^ is a testament from someone who, unfortunately, COULDN'T figure out how to do it right. Listen to the follks who COULD figure out how to do it right.

A couple glaring points form above:
1. If you ever start having problems with the units themselves it gets to be a nightmare real fast.
The "units" themselves are about the most simple fuel dispensing device ever made. Simple.

2. Also consider a non-computer controlled ignition and going back to a conventional distributor
If you make the choice to go carb, then you have to go to a non-computer controlled ignition system. Car won't work right at all w/the stock ignition and a carb.

3. Power can be made from TBI's they are just incredibly temperamental and spending the money to get away from it now is going to be a lot better for you
I got about 300 hp from mine. It cost me less than $2k...and it ran fantastic for years and years until I sold the car.

4. Finally i just wanna clarify that I'm not an expert by any means
Well....that part appears to be true.


It's easy to get a CFI to run GOOD. It's easy to keep one running good. They are a very, very simple and reliable system.

 

Well you can't blame me for my transparency but, yes I agree with you Tom, it is a very simple system. My problem was that every single time we tried to get the car to drive right it would backfire out of the manifold. And through the TBI's butterfly valves. I wish i had grown up in the 70s because vacuum wouldn't be such a foreign concept to me. Essentially my mechanic and I were finding that we would go and recap or reattach the vacuum lines and caps to the TBI, and while we were tuning the computer (We had Dynamic EFI), there would be either a small or large backfire due to experimenting with fuel tables which would blow vacuum caps off the tbi units and thus make them run wrong again! JoBY is a perfect example of someone who took these units and made them work but bear in mind I was 17 at the time and anymore than the grand already spent was obscene for my budget so we threw a carb on it and called it a day. Justification to my mechanic. He has been drag racing since the 80s and has worked with early Fuel injection extensively including the TBIs for GM. He said that he had never seen TBI's behave like that and after finding out that the 2 tbi units only flow a measly 600 ish cfm together i was about done with them. I''m sure if I had a good understanding of the units i could get them to work but at that point I had the renegade manifold, a cam, new heads, and 10.5:1 compression which wasn't making that computer happy with me at all. If i had just slapped the renegade on there and ported the heads + cam and chip tune like most recommended I probably could've got near your 300hp mark without upsetting the factory computer even. But considering the circumstances i didnt have the time or desire to make it work right. From what i read about them they are happy so long as you dont upset the original ECU. if you do that then you will be in pain.