VPC, 7M, FFIM, Lean Spike From Idle...

[BillyM]

...ok, some of you guys here know these systems upside down and backwards, whereas I'm just now getting into the 7m's. Could use a little help on this issue, especially from those who've gone the same route before (ie, VPC)

My 7m car, finally up and going, but having some slow-driveability issues...

I'm running a 7m, stock ECU, stock injectors, VPC /w stock 440 chip, RonR FFIM/w ISCV retained. All data is pulled with the LC-1 and Aux-Box (not hooked up yet). I just verified the TPS is OK, the vac-hose to the VPC pressure sensor is only ~8" long (that's what she said) and sofar my only other complaint of the running condition is a slight miss at idle.

The car starts fine, idles fine (albeit a bit low for my tastes, 600rpm or so) I get the car warmed up, everything going as it should, and my first throttle stab after idling, I get a vicious lean-spike that nearly kills the engine, then it pops right back where it needs to be.

...if I pull it up to 1200rpm, then give it a go, she's a-ok. ...runs really nice AFR's everywhere else. Closed-loop is working great at idle and cruising, going wide-open while cruising gives a nice, dependable enrichment that goes right where it should when boost comes in. (open DP sounds so sweet, by the way)

Everything is groovy EXCEPT that first touch of the throttle. Does anyone else see this? What do you cats think it could be? I'm going to check the VPC pressure sensor tonight and make sure the voltages are within spec, but other than that I'm lost man, got no clue...

Thanks in advance, all helpful posters get free beer at first chance. (You deals-gap folk keep that in mind, only 3 weeks till free-beer time.)

--billyM

 

[Justin]

What fuel pump?

When we still had the 88T I had this same problem. I did the 12volt mod and that totally cured the problem... Just my experience

 

[BillyM]

Walbro, I believe. I don't imagine it is having fuel pressure trouble, due to how it is acting, but I will definately keep an eye on it, thanks.

I'm going to get the full datalogger setup on it tonight/tomorrow. I have the Innovate LMA-3, will pull rpm, MAP, fuel pressure to get a bit better of an idea on it. (I currently only have the lc1 only attached to make sure AFR's are where they need to be.)

...and I have to say, 3000lbs + 7mgte = fun. 7 psi makes this car quick, I can't imagine what 18+ will be like. I REALLY look forward to having the quirks ironed out so I can start throwing on the new pieces. ...550's, and the matching VPC chip, AFPR, and the boost controller all waiting in a nice pile, ought to get it where it needs to be power-wise.

--billyM

 

[figgie]

you idle is too low.. needs to be 650 for manual or 800 for auto. Idle can be adjusted without touching the tb or tps. screw is on the side. Flat head screwdriver needed.

 

[joliroger4]

It depends how bad the stumble is. The stock setup by nature has a small hesitation when you stab the gas from an idle. If it is excessive, it is probably the TPS. It has two functions, to let the ecu know the throttle position and it also has a switch in it which will fire the injectors inturn acting as an accelerator pump to avoid avoid the exact symptoms it sounds like you have. It is possible for the contacts for this switch to wear out, but can still give the correct position signal to the ecu.

Bottom line, if it stumbles really bad off idle it is probably your TPS, but if it is a slight throttle response issue, that is normal.

 

[BillyM]

Figgie: I will bring my idle up once I get the datalogger on it in the next day or so, as well as double-check my timing settings.

Concerning the TPS... as I understand it, there is an IDL circut and a resistive pot within the TPS. If both are testing correctly, I don't understand how it could be bad. I did have to extend the wire harness for the FFIM, so I will test it at the ECU to make sure my connections are all a-ok and I did not introduce an unacceptable resistive load by extending it like I did.

Thanks again guys, keep them coming.
--billyM

 

[joliroger4]

Figgie: I will bring my idle up once I get the datalogger on it in the next day or so, as well as double-check my timing settings.

Concerning the TPS... as I understand it, there is an IDL circut and a resistive pot within the TPS. If both are testing correctly, I don't understand how it could be bad. I did have to extend the wire harness for the FFIM, so I will test it at the ECU to make sure my connections are all a-ok and I did not introduce an unacceptable resistive load by extending it like I did.

Thanks again guys, keep them coming.
--billyM

You are right, I don't get it either, but I put in a tps once that tested in spec and the car had a really bad hesitation. Put the stock one back in and everything was fine again. :dunno:

I extended the wires on my truck and that car has instantanious throttle response so I am willing to bet your extended wires aren't your problem.

 

[SevenMKIII]

What's your response knob set at on the VPC? Check this site it explains each knob in detail from HKS. Sounds just like what you're explaining if your TPS checks out okay.

http://www.alltrac.net/manuals/vpc6.jpg

-Chris

 

[3p141592654]

The problem with our Karmen-Vortex AFMs is that at idle the air-flow signal to the ECU is a very low frequency (30Hz or less), which means that with no electronics between the AFM and ECU you are looking at a minimum of ~30ms (the period of a 30Hz signal) before the ECU can tell that the air flow has increased.

If you throw crap in-between the AFM and ECU (like your VPC) then the off-idle lag gets worse because now you get at least 60ms of lag.

The stock ECU uses the TPS to minimize the lag. When the ECU sees the throttle move off idle it disregards the AFM signal and immediately adds some fuel to the equation.

Bottom line is that this is somewhat of an unavoidable problem with our setup. Your VPC has a response knob to try and workaround this, so make sure you optimize that.

 

[BillyM]

...like I said, it is only the first stab of the throttle. If I increase the response, I get a huge rich-stumble after the concurrent throttle stabs. If I decrease the response, I don't get enough fuel.

Pi', First things first, the VPC replaces the AFM, and I'd venture to bet that it actually reacts faster than the stock system, but you say that when the ECU detects a large TPS transition off the idle switch, that it disregards the AFM signal. ...in that case, it doesn't matter what kind of response it gets from the electronics.

I'm going to get the datalogger on it soon and get some honest-to-god hard-numbers and find the correlation between timeframe, map change, and throttle increase threshold.

...got my timing fixed, and going to see about popping the cap off the throttle body and getting it not-so ISCV-dependent.

--billyM

 

[3p141592654]

Sorry, I was describing SAFC type devices that go between the AFM and ECU. The problem remains, however, since the VPC generates a frequency that simulates the AFM. and the frequency is low at idle, no way to get around that first 30ms of dead time.

It is a delicate balance during that transition, and that is why you struggle between the two extremes.

 

[suprarich]

I had the same problem with my VPC untill I set the response a little better and mounted the pressure sensor better with the vac line pointing straight down.