Hi

First. Car is a corvette 1991 6spd.
Lt4 hot cam kit
Ported 113 alu heads
Headers

When cruising at low speed, and I go from idle to light throttle, AND if I cruise at very low speeds, the car starts to “jump” and I have to disengage the clutch and start over again.
I got the fuel Cells almost dead on at 128. Could this be solved with timing, or perhaps accel pump settings? I will keep trying to tune this behaviour out. I just want to know if someone experience the same strange phenomena?

cheers

ps: im using the stock timing tabel for now

 

Capture several instances of this behavior in a data log. Then look at the TPS%, SA timing and the BPW. The ECM may be switching between closed throttle SA and main SA. It may also be switching between single and double fire mode.

The data logs will show what is going on. It may also be a case of just trying to cruise at too low of an engine speed for this particular engine.

RBob.

 

Got ya..

How can I se in ( in my case datamaster ) if it switching between single and double fire mode?


Dont think that its to low crusing speed. The engine is nothing radical. And I cant downshift to a lower gears, as Im runing in first. This happend during "normal" stop and go traffic.
Dident have this probleme when it whas stock. But then again that fact berhaps dont mean a thing in this case heheh

Regards?

Anyone else experinced this phenomena?

 

i sure did. BUT that was day one before tuning on stock ECU and chip with full mods. in fact it was so bad the car would lay rubber as it bucked/surged. disengage clutch. once the 7747 went in and tune was improved it went away.

 

ok sounds like this mayby can be tuned away...?

thanks

 

I have the same problem and have been working on this as well. I have not been able to get the problem to go away entirely but have made some progress in smoothing it out. I'm running a mass air flow and when the LV8 is 90 or under and between 1000-2000 rpms the problem occurs. I'm also running a 6 spd. I have played with the spark advance in that range and by adding more advance the problem smoothed out. I'd be interested in knowing more about how this single to double fire deal works, never heard about it.

 

Been trying to fix this in $6E, $8D, and $58.

Good luck..

-- Joe

 

well I dident have this probleme when the car whas stock..

 

Neither did I ..

Ive been trying for years to figure it out.. I've got logs of JUST it doing that. No difference in S/A, etc.

-- Joe

 

this is strange....

What have you done to your car?

I got the hotcam kit and headers.

Thanks

 

look down..

 

I wounder what can cause this. My mods are pretty lame. And the cam is not radical ( hotcam ).

 

Maybe this is a dumb question but what is closed throttle SA and main SA? If it was switching between this and single and double fire mode how do you adjust it out?

 

In the $8D code (which is what devilfish is using) there is another SA table that is used during closed throttle conditions. It is SA vs. RPM whenever then TPS is less then a threshold. At low throttle settings it is possible to flip between the two tables. If the SA's don't match up then sudden changes in SA can occur.

The single fire injector mode is used whenever the PW drops below a set value. Double fire is the 'normal' mode, two injectors firings per cylinder firing.

Single mode is one injector firing per cylinder firing. The calculated PW is doubled and then injected half as often ($8D mask again).

RBob.

 

I'm running a mass air flow, so would the switching between single and double fire apply as well? How would I change where it switchs? One thing I have noticed is this does always occur at a light throttle position. If I bog the engine down in a higher gear which effectively raises the throttle position the "jerky behavior" goes away.

 

I had the same problem in my trans am w/ hotcam + ported heads and 24lb svo injectors but most every thing else was stock. Mine had a small lean spot if you were trying to run about 900 rpm or so and were in first gear with almost no throttle. It would start to buck like crazy. Once I got rid of the lean spot it was smooth. That was when the car was maf. I later switched to the 808 stuff when my maf went bad. I was amazed how smooth it was after the switch.

 

I have a Totaly Stock 1985 Camaro that does this. It didn't start Untill we Ripped out the 700R4 and replaced it with a t-5 about a Year ago. I think it has to do with the T/F and stuff since the prom was for a AUTO.

Currently I am in the middle of swapping out the 1226870 ecm for a 1227165 ecm and I am going To go with the 1989 ANZA Bin which is for a Manual Tranny

So I hope this will cure it.

Now If I can just find a use for my 870' ECM.

 

devilfish,
perhaps the disto's swing weights are sticking...

contact

 

Might look at the Decleration Fuel Cut Off, and Deccleration Enleanment stuff.

With a big cam might even have to look at the MAP filtering stuff. Or rely more on the TPS end of the DFCO things.

 

thanks for the tips

"devilfish,
perhaps the disto's swing weights are sticking... "



GRumpy:

"With a big cam might even have to look at the MAP filtering stuff. Or rely more on the TPS end of the DFCO things."

Could you explain more about the MAP filtering stuff. You got me there..??

 

Try a few small changes and see if it helps.

 

playing eith my hocammed vortec head formula and my new wideband :-)

Seems to me folks with hotcams have issues in the off idle and lite cruise areas 1200-1400 especially..

Widebanding it and using the romulator i found a high 15 low 16 AFR, if i tuned it hands on to 14.7 on my wideband the problem ceased. Also eliminated the stumble that i would have under a medium throttle rollout from a stop .....


try running it open loop and richening up the affected area if u dont have a wideband and see what happens...

later
JEremy
PS now someone give me time to tune my car with the wideband all the way ....please ....really.... im not joking

 

Yes I would if I just know what "MAP filtering means? Care to explain?

cheers

 

Devilfish,

I don't know what the MAP filtering is either but I took Grumpys suggestion and tried playing with the deceleration today and it has smoothed it out some. It's not perfect but it's probably one of those situations where a little of several factors are contributing. What I changed specifically is the "DFCO Enable if %TPS< X", this is in TunrePro. I changed it from 3.13 to 1.95.

 

sweeet! going to try that out. I wounder why that should differ between a stock car and a light modifyed one??

 

Well, i was thinking and since a larger cam and/ or throttle body is pulling more air into the cylinder you probably don't need the fuel cut off to kick in as early because then it will lean out for a bit and that's where the hesitaton comes in. I had some similar problems with power enrichment needing to kick in earlier(lower delta TPS). Let me know if that helps ya out. Do you know what your BLM's are when the hesiation is occuring? Mine are about 123-125 which could be contributing also. That's my next thing to dial in.