Hi car is 89 Corvette 165 ECM with a miniram, CNC'd heads and mild crane cam. I had a Super Ram and just switched to a miniram. Wow what a difference. I love the miniram. I have a tip in problem though wehn cold and in open loop. The car starts and idles fine but it you goose it, it has a stumble and sometimes backfires through the intake. I checked for vacuum leaks and everything looks good (16-17inches of hg). Timing is at 8 degrees and fuel pressure at 48 psi (WOT). Plugs all look good.

What do I look at for this? What do I change? Should I increae the idle speed when cold? Block learns at idle look good in closed loop. What can cause this?

Thanks,
Jason

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89 Vette 6 speed
CNC heads, crane cam, Miniram, headers
Soon to be 396 Small Block!

 

I have this same exact problem with my setup, and its not the timing. Your car is running to lean, so is mine. Bump the fuel pressure up to 60psi and that stumble and backfire goes away. But if you leave it that high for a long time the injectors dont like it(2 of mine took a dump).At first ithought it was a vacuum leak, it wasnt and I played with timing for a long time and never got rid of it. To totally get rid of the hesitation and backfire i unhooked the o2 sensor from the ecm and lowered the pressure to 52psi(this is with the vacuum unhooked). For some reason unhooking the o2 wire makes my car run a little richer. Of course this is just a bandaid. The car really needs to be reprogramed for the miniram.

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87z 383,afr 190's, crane hyd roller(224/230-.509/.528,112 sep),Ported and polished mini ram, 24lb inj, 3.42 gears, strange 12 bolt, tremec 5spd, , 1,3/4" slp headers.(stock 305 hypercrap chip), other stealth goodies.
Runs 14's all day long.

 

Hi,
My stock injectors won't take any more than 50 psi and then they begin to shut down and actually deliver less fuel. I have a new set of FMS 30lb units for the 396 when I install it. Glad to hear I'm not the only one with the problem I guess I need to richen the idle mixture and see what happens.


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89 Vette 6 speed
CNC heads, crane cam, Miniram, headers
Soon to be 396 Small Block!

 

Access the Pump Shot table in the PROM and modify it with about a 20% increase. If that doesn't work then jump to a 30% increase, then finally a 40% increase.

I had a similar problem and ended up using a 40% increase (changed the 25% values to 35%).

I am NOT saying that this will work for you ... but, it did work for me.

Tim

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TRAXION's 1990 IROC-Z
Best Time = 12.244 @ 112.51mph (1.778 60' / 7.819@88.32mph in the 1/8)
All Natural. No Force. No Drugs. Stock Bottom End. Stock Body Panels.
Gunning for NA 11's after installing Hooker LT Headers and reducing weight.
-=ICON Motorsports=-
Moderator: PROM board at thirdgen.org

 

Hi, do you have the address for the pump shot? My GMEPro epe file does not show these so I guess I need to do a direct edit. What are the decimal values? A percentage increase of idle fuel?
Jason

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89 Vette 6 speed
CNC heads, crane cam, Miniram, headers
Soon to be 396 Small Block!

 

 

 

Grumpy I disagree with you on the injector pressure. At idle, if I turn the fuel pressure past 50 psi the block learn goes way up and the car will hardly idle. At 60 psi it won't start (plugs dry). Maybe its just the multec's. The backfire happened yesterday while in closed loop and driving. I was driving normal shiftign from second to third. Slight hesitation...POP....and then all is good. My block learns are 126-130 at idle. If you change the injector constant would you change it to a lower value than 22pounds so the ECM adds more fuel?

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89 Vette 6 speed
CNC heads, crane cam, Miniram, headers
Soon to be 396 Small Block!

 

[QUOTE]Originally posted by 89vette:
[B]Grumpy I disagree with you on the injector pressure. At idle, if I turn the fuel pressure past 50 psi the block learn goes way up and the car will hardly idle.

You crank the FP up, and the BLs go up?.
BLs should go down.
If they go up that means your running low on fuel, which can suggest that the pump is shutting down. I'd be more suspect of a pump going south.

At 60 psi it won't start (plugs dry). Maybe its just the multec's.

Gosh, hope no one uses them in a boost situation of 15 PSI with a intial setting of 43 or so....

The backfire happened yesterday while in closed loop and driving. I was driving normal shiftign from second to third. Slight hesitation...POP....and then all is good. My block learns are 126-130 at idle.

If this was right during after the gear change might look at the timing, first.

If you change the injector constant would you change it to a lower value than 22pounds so the ECM adds more fuel?

Yep.

BTW, No need to argue, just go over the archives at the syty.org list, and research it your self.
Was about 2 years ago.
Cheers

 

Hi Grumpy. Are you Bruce Plecan from the GMECM list? I can make it pop by just goosing the gas in neutral so its not just after a shift. As far as the injectors go saying that I disagree with you was not a correct term. I didn't mean to come across that way. My fuel pump was replaced last year. It's proabably just my style injectors. I'm going to add 10% of fuel or so to the first 7 or so MAF table values and see what does. I'm running a gutted MAF and I hear they under report airflow. One thing I noticed is that for some reason the car starts really fast now as compared to the TPI or Super Ram set up.

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89 Vette 6 speed
CNC heads, crane cam, Miniram, headers
Soon to be 396 Small Block!

 

89,

Before making any PROM changes I would suggest 3 things ....

1) Doublecheck your initial timing (with the EST signal disconnected of course). This is of paramount importance because you don't want a dumb timing issue affecting your car.
2) Verify that the FP at WOT is steady. Hook up a FP guage and duct tape it to your windshield. Go out with a friend and have the friend watch the FP over a brisk WOT run. It should be steady and not decreasing.
This will verify that your pump is up to the task ... yes, I know you just replaced it but it doesn't hurt to verify it.
3) Make sure your car is in a good state of tune. Good plug wires, good cap/rotor, good plugs. An ignition issue could easily cause this problem.

Tim

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TRAXION's 1990 IROC-Z
Best Time = 12.244 @ 112.51mph (1.778 60' / 7.819@88.32mph in the 1/8)
All Natural. No Force. No Drugs. Stock Bottom End. Stock Body Panels.
Gunning for NA 11's after installing Hooker LT Headers and reducing weight.
-=ICON Motorsports=-
Moderator: PROM board at thirdgen.org

 

All good advise Tim. I did double check the timing and had it from 4 to 8 degrees. Its set on 8 now. I also played with fuel pressure 42 to 50. It's at 50 now. I didn't do the WOT thing with the fuel pressure becasue the problem was a low speed thing. I'll do it though.

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89 Vette 6 speed
CNC heads, crane cam, Miniram, headers
Soon to be 396 Small Block!