Question : What happens when ?

[Urbman]

Hello All
I am a new member. Thanks to all who have put so many good things together on this site.

Could I trouble you for a pointer or two regarding a flow chart as to what happens during a normal start to a normal warm up.

I am trouble shooting a 1998 LA1 powertrain. My trouble is that the engine
seems to run well up to or around warmup and then disintegrates into
a wheezing mass that will backfire wickedly at anything over aproximately
1200 rpm.

So far...
1. New fuel pump and filter
2. New fuel pressure regulator

measures 42 PSi key on engine off, idles around 36psi
snap open the throttle, 42 psi maybe a bit more.

3. New plugs, new wires
4. Static check of coils yields .4 ohm primaries 5.6K secondaries +/-5%
5. New catalytic converter

The behaviour is pretty consistent 5-10 minutes of bliss
(quick start, smooth idle, smart acceleration ) followed
by a stumble and then limp home at 1000rpm any more throttle and
a substantial backfire occurs. If I persist ... a stall and a restart
is required.

I will try an ICM tomorrow but would welcome any other suggestions.

It occurs to me that I don't really know whats going on as the engine warms up. Things like ...
When is the purge valve opened ?
When is the egr used in this cycle ?

I looked around the site but I seem to have missed it

Al

[carbon]

Check engine light? Codes?

Is the O2 sensor in good shape? The engine falls apart when it should be going into closed loop...

It tries to idle down but the IAC passage maybe plugged choking it?

Just some thoughts...

[bszopi]

Since you mentioned its good until it gets to normal operating temp, I would definitely check the O2 sensor.

[Urbman]

Hello Carbon
The 02 sensors are new. Replaced as part of this exercise.
Trouble codes are not set unless I force the throttle and get a sufficient backfire.

With respect to "Closed loop" ...
I thought that closed loop was a steady state condition. Throttle position constant,
engine load constant. I was under the impression that under acceleration this motor
is not in closed loop.

Could I trouble you to confirm. I am heading out and will look into the IAC before I start
on the ICM swap.

Thanks for the reply

Al

[bszopi]

Closed loop is when the O2 sensors are controlling the fueling. When in open loop, fueling is done by default table values. Once the engine is up to temp, the O2 sensors beginning monitoring and adjusting the fueling tables as required to maintain a stoich condition.

[Urbman]

Hello Bszopi
Thanks for the information. So I am clear on this ... Once I am at operating temperature
the 02 sensors are quick enough to be part of the fuel injector duration calculation
under acceleration.

Would it be likely that an old and new o2 sensor would behave the same way.
My symptoms haven't changed since they were changed out.
Thanks for the clarification.
Al

[Urbman]

Minor update...
When I went to replace the ICM I noticed that the cable to the crank position sensor
was wedged against the pipe to the EGR valve. Closer inspection showed that the
one of the wires is damaged (melted insulation). The cable runs around the back
of the engine and terminates at a connector mounted on the block.

The connector from the crank position sensor plugs in here.

I tried my local napa place but they only sold connectors with pigtails.
The wire color was also a bit off (not yellow and purple).

How do you disassemble these connectors ?
Where can you get pins for these connectors ?

Thanks
Al

[Superdave]

I'd go to a salvage yard and yank a whole CPS harness off another 3100 or 3400 car. Should be very cheap. Repairing that harness is not advised since any difference in the wire can cause a change in recieved pulses to the ICM and delivered spark advance.

I learned that the hard way.

[IsaacHayes]

yes there has to be so many twists per length of the wire or it can pick up interference.

[sprucegagt]

Hello Bszopi
Thanks for the information. So I am clear on this ... Once I am at operating temperature
the 02 sensors are quick enough to be part of the fuel injector duration calculation
under acceleration.

Would it be likely that an old and new o2 sensor would behave the same way.
My symptoms haven't changed since they were changed out.
Thanks for the clarification.
Al

Actually closed loop occurs when the O2 sensors are warmed up. It's not dependant on coolant temp. Heated O2 sensors (3/4 wire) usually warmup somewhere between 30 seconds to 2 minutes after start up. All depends on outside temp (colder = longer).

[Urbman]

Another update ...
Just got back from the parts store. To my disappointment the leaky hose is too expensive
for me. The hose connects a pipe from the block to a tee fitting and what I imagine are
the heater hoses.

I am going to try and replace the o rings in the fitting. Are there just two ?
(I am having trouble seeing into the fitting)

Al

[Urbman]

Update
Hose
The connector yielded 2 orings. I didn't know what size the originals were. I replaced
the originals with a pair of 3/32 orings.

Cable
I obtained a new cable (7x crank position sensor to ICM, 80bucks ).
I replaced the cable.

I would have gone ahead with this but I was still troubled by the symptom
that it goes to hell in a hat basket after warmup . I replaced the ICM
at the same time. ( The NAPA folks indicated that I couldn't return the
ICM, only exchange it so what the heck )

All seems reasonably well at this point except that my OBDII scan tool
indicates that some "I/M Readiness Monitors" vaven't been performed.
The "monitors" in question are ...
1. The catalyst monitor
2. The heated oxygen sensor monitor
3. The evaporative systems monitor

Do I have to do something specific to restart these monitors ?
(1998 pontiac transport 3400 SFI)

[bszopi]

http://www.2gstratus.org/forums/index.php?showtopic=32354

See if that helps (post #4 seems to explain a lot). Might also try disconnecting the battery overnight and see if that does anything for you. Glad to hear it seems to be heading in the right direction, though!

[Superdave]

Actually closed loop occurs when the O2 sensors are warmed up. It's not dependant on coolant temp. Heated O2 sensors (3/4 wire) usually warmup somewhere between 30 seconds to 2 minutes after start up. All depends on outside temp (colder = longer).

Not sure about OBD2 but in OBD1 $A1 code, Closed loop is on 3 seperate timers (cold, warm, hot) and min/max coolant temps for each. After a certain amount of time the ECM assumes the O2's signal is useable.

[sprucegagt]

I guess it's different with OBDII.