Got an '88 with headers, small cam, TPI Systems throttle body and intake manifold, Mini Ram.
Got everything running good except idle, I have checked for vacuum leaks, none. I changed IAT, IAC, ECM, Electronic spark conmtrol and ignition module inside the HEI distributor. TPS is 0.54 volts at idle.
At idle in park, after going closed loop, the timing is jumping up to max advance at 42 degrees....with timing wire diconnected, the timing goes down to base idle, at 6 degrees, and the rpm is 650-700...runs perfect.
As soon as the timing wire is connected, the timing goes max advance, and idle increases to 1300-1500. I cannot find anyone locally, mainland or googling forums to tell how to find what, or why the computer is commanding full advance. Can anyone here please tell me, or if they know of this problem? Where do I look? What do I troubleshoot? Is it something telling computer? Signal missing?
Appreciate any help....

 

Maybe this will help. Look at the chart(s) below. This is your timing table. RPMs are on the left rows and LV8 is across the bottom columns. LV8 is a measure of load on the motor (at any given moment).

The chart on the right is how much MORE timing is added when the car is cold. Again LV8 across the bottom. This time temp is the control for rows.

Add the two tables together to get total timing at any given moment. (Note these tables can be altered by performance tuners.)

At 800 rpms, during the warm up cycle, the engine will flucuate betwee the 48-60, and even 80 LV8 columns. Until it gets to 68 degrees running temp, you'll have max advance.

When it gets above 111 degrees, you finally drop all spark adders and only look at the main timing table. Notice, even with the slightest rpm increase you might see timing values as high as 37 or 40 degrees. If at 600rpms and hot, you should see 20-deg. If not, your timing is wrong, your balancer is wrong, or your programming has been altered.

I think I also suggested some other possibilities in another thread you started including faulty ESC module and distributor wiring.

 

Sir,
Appreciate your help, but not sure I understand these tables, columns are up and down, rows are left to right...so where is "across the bottom colums"?
I did look at your suggestions from previous thread, and changed the ESC and Ignition Module in the distributor. The wiring all checks out good.
After it warms up, temp is at 208 degrees, and timing is still at 42 degrees advance...so it's not comnforming to tables. I cannot get it to idle at 600 rpms at idle when hot...cause timing is maxed...if I disconnect timing wire it will idle at 600 rpms when hot...so my timing is not wrong, balancer is not wrong and I do not know about tune in chip...
Something between ESC, Computer and Ignition module is driving the timing to max...unless you think that the computer thinks it's in warm up per these charts all the time? IDK....very frustrating

 

......... Check the engine coolant temp sensor ... its in the front of the intake manifold ... there is another temp sensor in the pass cyl head for the gauge ... new ECT's are less than $15 ........

 

That's a great point. If the ECM thinks the engine is cold ALL the time, it's going to (try to) add 20-deg of timing ALL the time.

 

"Across the bottom columns" was a mis-communication. I meant the same thing as going right-to-left.

The sensor you should check/replace is the one below the thermostat housing. The temp you see on the dash is obtained from a different sensor (by the dipstick). The coolant temp monitored by the computer is the one below the thermostat. As I mentioned (and C409 brought up), failure from that sensor to show a warm-up would lead the ECM to think its cold and never heats up.

So, in the timing table, it would always add those lower 20-deg add'l advance rows from the right-hand "Spark Advance by Coolant Temp" table.

This would cause what you are seeing. It was something people have fought before and is something I should have thought of!!!!

Unless you have a data logging program (or scanner?), you can't see the temp the ECM see's. Kindof a weird, confusing system, but that's how it works.

 

Thanks both of you...
After learning your timing maps, I thought the same thing, ECM thinks cold and not warming up. I tested the coolant temp sensor and it seems to move accordingly on the ohm meter, but I'm going to replace it anyway and see what happens.
It is getting a 5 volt signal, and when disconnected, ecm will have no voltage sensing hot? Still no difference.
I have a INNOVA 3140 scanner, but will not allow me to see what the ecm see's cause it's OBD1...try again tomorrow, and thanks again....

 

Another possibility is your IAC motor Vs throttle adjustment. If your IAC count is already at its minimum position and you connect your spark control wire, the engine RPM will naturally increase but your IAC cannot pull the idle back down. This will also cause your spark advance to increase compounding the problem. There is an IAC adjustment, then TPS adjustment proceedure in the service manual to correct if this is your problem.

 

If you have a voltmeter you can measure the voltage at the sensor. The ECM supplies 5V, so that's what you will see if you remove the connector from the sensor and probe the connector. If you see 5V there then the wiring is good.

The resistance of the sensor is EXTREMELY nonlinear. It drops very slowly with slight increase in temperature until it gets hot; then it changes a lot. You can measure the voltage with the connector connected by sticking a pin through the insulation of the wire. The engine doesn't have to be running to do this test, but the ignition does need to be on.

 

hey greg that apyp picture you posted? what does the apyp mean? does it apply to a 91 convertible too? (still trying to figure out what all the stuff means inside my tunerprort and how to use it...)

 

What chip are you running? What is your IAC count at idle? My assumption is the chip is commanding that much timing and the IAC is reading zero so it cannot slow the engine down. You could have the tuner who made your chip adjust the idle timing or at least tell you what it is commanding.

 

Brian,

I think I suggested that the OP pull the ECM and look for a piggy-back chip (like we use). This was in another thread after he posted this problem a first time. I don't think the OP knows who/where the engine was tuned. He is a subsequent owner, right???

OTOH, I didn't know if he would be able to tell if an entire prom had been burned. (and if so, how to read it) Seems like I've seen where some tuners can burn a prom vs using the piggy-back chip.

Obviously, there'd have to be a mistake in burning for full advance to exist at a low rpm. That's why we've headed down the road of alternative causes.

 

I don't know if APYP stands for anything but it is the stock Corvette XDF file as displayed on TunerProRT's website. I used it for comparison. Brian modified a few things from it -- and I used it as my starting point.

The one for a 1991 may or may not be called APYP as well. I can say that speed density is a whole different animal than my 1989 MAF bin. Since I haven't looked at a 1991 bin (SD), I can't say how the tables I posted compare to yours.

 

All you folks here have been very helpfull and smart on a lot of things, really appreciate all. I took it to a local shop and put it on his snap-on scanner and it showed one code for coolant temp sensor. Ran good going there, but could hardly run on the weay home. It will not come off idle, stalls and pops, then stutters as it gets going, once I get to cruise about 25 mph it goes enough to get home. Light is on.
Still have timing/idle problem, looked at and tested all sensors, including testing all the wiring, one at a time...replaced the plug and wires for the coolant temp sensor, from firewall to sensor. It was intermitently showing open. I found that Jim at TPI specialties is who burned the chip originally and been in touch with him, he been very helpful also. Will try one more time with snap-on scanner, then send chip and computer to Jim to see if he can find anything...I cannot find anything on car as of now....real close to a carb mod, dump it, or part it...

 

Some of the info from scan was;
EXH O2= Lean
TPS=0.54
O2= 484
IAC=71
Desired Idle=888
Coolant=85 C
MAT=40 C
MAF=14
LV8=63
Spark Adv=27

 

If its idling at 1300-1500 rpm as orginally described, the extra spark advance is likely due to hiway mode spark. This will add an extra 8 degrees or so to the main spark table depending upon the bin.

 

IAC = 71 is a big part of your problem. That should be down around 15. I'm surprised Jim didn't catch that. A high IAC is an indication the idle set screw needs adjustment.

If you turn your idle adjustment screw (opening the throttle blades) until the idle speeds increases...the IAC will close and stop trying to do the work for that lack of idle air. (When the commanded idle can't be reached, the IAC opens to let more air in.) That's what's happening when you get the long, high revs at the start.

IAC should sit around 15-20 when the car is hot and the fans aren't running. 20-30 with the fans.

According to your scan, the spark advance should read 27 at idle with the EST wire plugged in. These EFI's are higher that some people might expect.

 

Unless TPiS screwed up the parms, that's not likely for a car idling -- at rest. That's because there's a minimum MPH for highway mode. Setting that parm below 50-55mph would be a mistake.

 

;--------------------------------------------------
; HIWAY SPARK CONTROL
;
;--------------------------------------------------
LC17B FCB 120 ; IF COOL <= ,DISABLE HIWAY SPK MODE ((Val+ 40)*256/192)
LC17C FCB 100 ; IF LV8 > 100 ,DISABLE HIWAY MODE
LC17D FCB 36 ; IF RPM < 900 RPM DISABLE HIWAY MODE
LC17E FCB 5 ; IF CONDITIONS PRESENT > 5 SEC ,ENABLE
;--------------------------------------------------
;--------------------------------------------------
; HIGHWAY MODE SPARK ADVANCE vs LV8 (load)
;
; TBL = SA * (256/90)
;--------------------------------------------------
LC17F FCB 23 ; 8.0 32
LC180 FCB 23 ; 8.0 48
LC181 FCB 23 ; 8.0 64
LC182 FCB 11 ; 3.8 80
LC183 FCB 11 ; 3.8 96
LC184 FCB 0 ; 0.0 112
LC185 FCB 0 ; 0.0 128
LC186 FCB 0 ; 0.0 144
;--------------------------------------------------

 

I understand by your display of (assembler?) code above, HWY spark mode does not have a mph control. Without it, that means (as you suggest) tons of add'l advance would be present at all times -- IF you activate it.

In the stock bin, temp is set above 300-deg to prevent activation of spark mode. If activated, I would have expected it to work in conjuntion with HWY FUEL mode which does have a MPH parm. If the two functions did NOT work in tandem, it wouldn't make sense to EVER activate it. That's because it would always be active vs only on the HWY.

We (you and I) can see the parms, but do we know there's no "system-level" code that connects HWY fueling with the HWY spark function? If you're certain there's no connection, leaving it inactivated would seem necessity -- as it would be flawed. I would concur -- as you seem to imply -- that the HWY spark function would be flawed without a MPH parameter.

In my earlier readings, I thought HWY fuel and spark functions were disallowed by the EPA as "artificial" means to satisfy governmental fueling standards. (I thought "poppy-****" when I read that -- if true)

If the government was directly/indirectly responsible for the suppression of this "technology", that's why I thought parms inactivated it from the factory. If there's no connection between MPH and fuel spark advance, the system would appear flawed -- and inactivated for THAT reason.

Please comment!