idle & misfire

[skyhigh4by]

Ok I asked a similar question several months ago but still haven't really sorted this out.

I have a tbi 350 w/ holley TB, salad bowl mod, Eddy intake, .450 lift cam, stock heads, headers. Fuel pressure is 17 psi, vacuum @ idle 21in/hg, timing 1-2 degrees, no knock sensor, no egr, heated o2. Its data logged, tuned, runs good, decent power, and excellent fuel milage for being on 38" tires. Mated to sm465.

Now for the problems... Start it cold, idles 1100 rpm until warm then down to 800 rpm(good) drive it a little, put the clutch in and idles at 1100-1200 rpm. keep driving, put clutch in and idles at 800 rpm. drive a little more, put clutch in and idles at 1200 rpm again. shut it off, start it up and idles at 800 rpm. drive it some more and idle will sometimes come up to 1200 rpm and sometimes stay down at 800 rpm. its totally unpredictable. the only certainty is that if i shut it off and start it again the idle will be at 800 rpm until I drive somewhere. I just want the idle to always come back to where it should be.

The other problem which I don't know is related or not is that when sustaining load driving up a long hill in say 4th gear it will have a very slight misfire that can be felt but not really heard. A passenger would never notice it but I can feel it because Im so used to it being there.

I think the fuel curve in the tune could be adjusted slightly because its a little rich at idle and lean up top but overall the plugs are a nice clean tan colour. I bumped fuel pressure up until I got a happy medium between rich idle and lean top end.

TPS checks out, CTS checks out, IAC is new, MAP sensor checks out, Knock and EGR disabled in tuning.

While data logging at idle I have noticed that the IAC counts don't really settle and seem high to me.

Ideas please....

 

[dyeager535]

You running VSS? Sounds like a potential VSS issue if not.

If it's tuned, why did you need to bump pressure up that high? Should be no reason for it to be fat/lean vs. good across the board.

 

[skyhigh4by]

No VSS. 208 t-case. Engine is in 82 p/u. The harness is from painless.

Im on the 3rd prom from tuner. There was still some more tweaking needed to the fuel curve but now I haven't heard from the tuner in a fews months.

 

[dyeager535]

Run VSS. Unless you have a particular reason you can't, do.

You can probably find some posts here about various experiences with TBI and VSS, but it's definitely tied into idle.

I'd hate to have you do a bunch of work adding it and not solve the problem, but the reality is, as long as you are running a stock speedometer, you are talking about the buffer, power, ground, and the third wire to pin A10 on the ECM to rule it out as the problem.

TBI seems to be less sensitive to VSS problems than TPI, but I've run without VSS intentionally before, and it took a bit of starting/stopping, but eventually idle started to "hunt", and it was obviously an issue. Plugged VSS back in, problem immediately gone. Never threw a code either, go figure.

 

[skyhigh4by]

Run VSS. Unless you have a particular reason you can't, do.

You can probably find some posts here about various experiences with TBI and VSS, but it's definitely tied into idle.

I'd hate to have you do a bunch of work adding it and not solve the problem, but the reality is, as long as you are running a stock speedometer, you are talking about the buffer, power, ground, and the third wire to pin A10 on the ECM to rule it out as the problem.

TBI seems to be less sensitive to VSS problems than TPI, but I've run without VSS intentionally before, and it took a bit of starting/stopping, but eventually idle started to "hunt", and it was obviously an issue. Plugged VSS back in, problem immediately gone. Never threw a code either, go figure.

How would I go about installing a VSS in the most sensible and cost effective manor?

 

[dyeager535]

Are you running the stock cluster?

I don't recall if '82 was first year, or '83 for VSS. Does your truck have cruise control? If there is a box on the fender with a speedo cable in and a speedo cable out, you have the old style cruise and won't have VSS. If you've got cruise and only one cable to the t-case from the firewall, truck was built with VSS. I'm *guessing* up until 1986, if the truck didn't have cruise as an option, it didn't have VSS. But pretty sure VSS will still work on those speedometers, just need the buffer to bolt to it.

Easiest way to run it assuming stock speedometer is find someone selling the speedometer and the VSS buffer, and buy those, if yours aren't correct. That would be from whenever they started, to 1989. (for the same body style of course)

Obviously going to have to remove cluster to look at what is or isn't already back there, if you haven't already.

 

[skyhigh4by]

Are you running the stock cluster?

I don't recall if '82 was first year, or '83 for VSS. Does your truck have cruise control? If there is a box on the fender with a speedo cable in and a speedo cable out, you have the old style cruise and won't have VSS. If you've got cruise and only one cable to the t-case from the firewall, truck was built with VSS. I'm *guessing* up until 1986, if the truck didn't have cruise as an option, it didn't have VSS. But pretty sure VSS will still work on those speedometers, just need the buffer to bolt to it.

Easiest way to run it assuming stock speedometer is find someone selling the speedometer and the VSS buffer, and buy those, if yours aren't correct. That would be from whenever they started, to 1989. (for the same body style of course)

Obviously going to have to remove cluster to look at what is or isn't already back there, if you haven't already.

It originally had cruise w/the 2 part cable which I removed when swapping in the FI motor. I used a crate motor and stand alone wiring/computer setup when doing the conversion. Its about as bare bones as I could make it. Theres far less clutter under the hood now with the EFI compared to what was under there with the carbed setup.

The stock speedo "works" but not great. It really bounces around a lot and is out some because of the gear and tire changes Ive made. Ive gotten really used to it but use gps when i really need to know my speed is spot on.

Is there some kind of VSS sensor that could maybe attach to the t-case where the cable comes off. Like a piggy back kind of thing?? Maybe just wired for power source and feed to computer?

EDIT- http://www.jagsthatrun.com/Pages/SpeedSensors_Speedometer.html
#2PRS looks like what I need??? Just an inline sensor, then figure just wire it to the ecm if I can find the wires somewhere in the harness.

 

[dyeager535]

PRS looks about right. It is 2K PPM that the early GM ECM's need.

Is the ECM you are using the 1227747 (or replacment PN)? If your speedometer is acting dumb, an '83-89 speedometer will have the buffer. Personally think that is an easier solution, since that keeps all the wiring in the cab, and if your speedo now doesn't work quite right.

I suspect in regards to VSS on TBI, the actual speed is much less important than the ECM knowing that the vehicle is slowing down.

 

[skyhigh4by]

Yes Im using a 747 ecm.

So the 83-89 speedo has this buffer that is a VSS and it would just have wires on it that I could splice into my harness? What would I splice it in to on my harness?

 

[dyeager535]

Yep, that's it. The VSS actually "unbolts" (it's one small screw) from the speedometer, but if you pull the cluster carefully, you'll find the buffer hanging off the back of the speedometer. From my experience I'd go with the lowest mileage speedometer you can find...I had the one in my '83 start acting up, I think it's just failure related to age and use.

Depending on the VSS buffer (the truck ones changed at least once through the years, all the same generally though, and I believe ones in cars similar vintage were the same as well, except many cars went to different VSS setups earlier than the trucks, if you are looking for donors) there will be a minimum of three wires: Power, ground, and the output that will go to pin A10 on the ECM.

I can't remember how the VSS is wired to the chassis, but I'm no fan of splices if I can help it. Being OCD in this regard, I'd find a donor and strip the VSS wiring out of it all the way to the ECM vs. splicing. Additionally, you might find that the power/ground are also separate plugs that will keep you from having to do any additional work on the fuse/ground sides, they might plug into what you already have. If it were me, I'd depin the donor A10 wire from the connector so that you could just plug it into yours.

IIRC, some of the aftermarket setups still run the systems like the trans is in park to eliminate the VSS problem, so that might be something that would need addressed in your case. IIRC it's normally referred to as "grounding the park/neutral" circuit or switch by many.

 

[wheels87k5]

#2PRS is what you would use. I went to an electric speedometer (Autometer 1489) and use the #2PRS. It is set up for the 7747 ECM pulse. I was able to use the wires from the VSS and the #2PRS to hook up to the speedometer and it works great.

Connect the #2PRS to the T/C. Oppisite end is set up for a cable for cruise control or backup for OEM speeodmeter. The unit has 2 wires. One goes to ground and the other to the ECM. Easy install.