96 S10 2.2 Idle Issues - Pulling my hair out

[wickedude]

Hopefully someone can help me out with my 96 s10 2.2 5 speed. I dont know where else to look. Here is the deal.

Start truck up (cold start) no problems at all. Once truck warms up and is shut off (off for maybe 10 minutes) then it starts up just fine. However within a few minutes sitting at idle it will start to die, sputter, and usually stalls. Always starts right up after. If you give it some gas when it starts sputtering then it goes away and does not reoccur. No problems while driving, just at idle.

Sometimes it will not die but will catch itself and the rpms shoot way up then come down after about 5-10 seconds.

I gave it a full tuneup not long ago (new plugs, wires, ignition coils, fuel filter, pvc valve)
I have replaced the TPS, IAC, and the fuel pump (for a different problem). Fuel pressure is staying constant at about 45 psi.

One of the old wires had alot of resistance and when I replaced it with 4 new wires the stalling was not as bad which leads me to some kind of ignition problem. Also the last time it did this it threw a P1441A EVAP System during Non-Purge. I disconnected the purge valve so nothing could get into the intake and it still stalls out. Scanner gives no codes or failures when it happens.I think the stalling caused vacuum issues with the purge valve.

Can anyone help me out? I dont know what else to try. Im using a Matco ProScan Plus scanner (OTC Nemisys). Thanks

[discnik]

Have you watched the data stream for the Mass Air Flow sensor ?
What are the fuel trims doing during this episode ?

Just some thoughts.

Also if your are a professional what do you think of the OTC scanner ?
We use the Snap-On Solus.

[wickedude]

On this engine there is no mass air flow sensor as I believe it was from 94-97 on the 2.2s. What should I be looking for as far as the fuel trims? Ive never been really able to catch them as its happening.

Overall im pleased with the proscan. I am not a professional, just a do it yourself kinda of guy. I love learning about cars and have built a couple racecars.

[discnik]

On the Data screen is there a "STFTrim" parameter ? If the reading is positive , above 0, the fuel is rich. If the reading is negative, below 0, then the fuel is being reduced to the injectors.

I was thinking maybe Engine Coolant Temp. sensor.

But that should have thrown a code.

Have you put a fuel pressure gage on the engine as well to see what the fuel pressure is doing during this ?

Comming off of the Air Box, there isn't a MAF sensor there ?

[wickedude]

Ok so your just after the short term fuel trim. I will get the scanner on there when it happens and get the numbers.

The coolant sensor is also new as I replaced it awhile back because the connector got damaged. Indeed there is no MAF on these engines. In 98 they used them on the 2.2s but not on the 96's.

I probually shoud have mentioned that both o2 sensors are new as well.

[wickedude]

Ok been playing around today and got some more info. First off fuel line pressure stays constant at just below 50 psi at the rail when this occurs. No drop offs or changes.

It appears that this occurs when the truck goes back into closed loop mode. When it starts up it is open of course, but the problems start when the scan tool says its going into closed mode. At this point my short term fuel curve jumped to -18.8% and the IAC count position goes way up to 60. Spark advance is slightly retarded to 15 deg. Engine Load jumped up to 100%. Somehow it is getting way too much fuel. Once it died and the other time it shot the idle way up like it does at which time I saw it shot back into open mode and stayed there idling and had no problems. This time it produced a new code P0507B along with the old code P1441A

Thus this is occuring when going to closed mode but only after the truck has been started once and warmed up some. And thus far I have produced 2 codes.

P1441 Evaporative Emission (EVAP) System Flow During Non-Purge
P0507 - Idle Air Control System RPM Higher Than Expected

IAC and TPS are new. Any ideas?

[discnik]

Ok, watch the Manifold Absolute Pressure sensor voltage if the MAP sensors voltage goes up without depressing the accelerator pedal it is telling the PCM that engine load has increased when in actuality it has not. This will cause a rich condition like the one you saw. The fuel trim went negative because the O2 sensor told the PCM there was too much fuel and not enough oxygen for a 14.7 to 1 air/ fuel ratio.

Did you clean the throttle plate when you installed the IAC ?

Let me know or Private Message me.

[Blue Bowtie]

The MAP is used in both Open Loop and Closed Loop Modes, but watching the input is a good idea.

You might also want to verify a reasonably correct signal from both the CTS and MAT. You might especially want to look at the O² signal.

Yes, the '96 has no MAF, but is strictly a speed/density scheme of operation. This requires an accurate MAP input, accurate MAT and CTS, and accurate O² signals. Since these engines also use the highly suspect "Flexi-Joint" on the exhaust manifold, make sure the O² sensor is not exposed to dilution from external air leaks.

The fact that it all goes south as soon as the PM hits Closed Loop should be a big indicator that one of the inputs is way off, but not quite enough to be out of tolerance and set a DTC.

[wickedude]

Looks like we are getting somewhere. opcorn:

Ok the CTS is new, replaced this summer, I accidently broke the connector. It seems to quickly change to temps and appears accurate.

The MAT, or IAT as I am used to, is not new. I did notice that it appearred a little sluggish but then Im not entirely sure what it should read. What are some normal numbers? I have a aftermarket air intake and the temp read about 100 degree lower than the IAT. I would assume that the IAT should hang pretty consistant with the outside air temp should it not?

As for the MAP I did notice that upon starting the engine load was 26% without depressing the pedal. Shouldnt it be only about 3% or so? Can someone tell me how much vacuum I should be seeing from the MAP at idle for this engine?

Thanks guys!!

[discnik]

Check your PM.

[wickedude]

Checked the values on the sensors.

My MAP sensor at engine off is 4.18 volts, which seems about right as im at 3800 feet here. Engine on it is about 1.2 volts at idle. The thing that seems strange with the MAP is that as soon as I start the engine the engine load is 23-26%. Is this normal?

CTS and IAT both are on temperature wise and react quickly to changes.

One I did notice the last time it stalled was that it threw a failure code (Did not give a SES light though) of P1406D. P1406 is egr valve pintle position circuit as the description. I have pulled the EGR awhile back and cleaned it, not much carbon was there.

Is it possible I just have a sticky EGR valve when it gets warmed up?

[discnik]

Check your PM.

[wickedude]

Pulled the EGR valve and cleaned it up. Did not appear to have much carbon and the valve moved fairly smoothly. Im fairly certain that the MAP sensor is the issue now as when the surging start the engine load and map voltage so way up and bounce all over. I have a MAP sensor on the way so hopefully that fixes it up. BTW what is a normal amount of vacuum in these 2.2s? I am only getting about 15 in.

[discnik]

Normal vacuum should be 19 - 21 in. HG.
Let us know if the MAP sensor was the culprit.

[wickedude]

I replaced the MAP sensor and the truck ran better and the fuel curves leveled off. However the truck stalled out again last night the second time I started it as it did before.

Man im bummed. Any other ideas what could be the problem ?