Overnight Cold Start Problems

The Coolant Temperature Sensor is bad!

It went bad in the cold range, and sends a signal to the ECM telling it the engine is cold. Almost like having a stuck choke.

Refinish King

To Refinish King, Thanks for your reply. Let me get this straight, you are saying that the new coolant sensor has failed during some point BEFORE full / total operating temperature was reached and this is why it works, (while connected), below 100 degrees or something like that?

Did you change both temp sensors or just the one that feeds the gauges/light?

This is a 1993 5.7 litre TBI injected engine with 189,000 miles on it.

I have replaced the O2, knock and coolant temp sensors as well as cap / rotor / plugs.

I have NOT replaced the ignition coil, spark plug wires or any MAPP or MAF sensors / sending units.

Each morning I have to re-connect the coolant temp. sensor to start the engine.

After the temp. starts to climb, (100-110), the idle starts to get weird, I dis-connect the coolant sensor, the idle smooths out and I have no starting problems for the rest of the day.

I need to mention a few other things...It takes 4-5 tries to start in the mornings-the IGNITION TIMING is 11-12 deg. advanced @ 600 rpm in park.

The vehicle runs fine with no knocking or pinging on 89 octane fuel.

Any thoughts on fixing this problem? ============= ============= QUESTIONS 4-U :

What happens if you unplug the coolant sensor imediately after starting the engine?? (instead of waiting for the weird idle to show)

What happens if you plug the coolant sensor back in AFTER the engine is running at normal operating temperature? (which is well above the 100-110 you posted)

Could you give a leeetle better discriptive of "weird idle" ???

.............. Do this....

FIRST THING IN THE MORNING..... go through your normal proceedure....

BUT..... instead of unplugging the coolant sensor when the engine starts to warm up, unplug a vacume line on the intake. (the smallest one you can)

Post the results.

............................. MY THOUGHTS??

The coolant temp sensor is working, and the circuit is good all the way to the ECM.

Once closed loop hit's, or the O2's start comanding fuel-air changes, the "weird idle" symptom shows up.

Most likely...... Unplugging the temp sensor is makeing the computer detect a cold engine scenario, and it's opening the IAC motor, and adding fuel, makeing the "weird idle" symptom go away.

And........ With the temp sensor plugged in, the ECM IS SEEING the eng temp rise, and is compensating by adjusting fuel-air.

Which tells us............ the entire temp sensor circuit is MOST LIKELY doing it's job.

No Gaurantees..... but without some circuit and component testing, that's the best you can deduce at this time.

I'm think'n you got something gummed up with carbon. IAC, EGR, O2, Vac ports.

any whoooo....those are some thoughts.

post yer results, and go from there.

~:~ MarshMonster ~not always rite.....but sometimes not wrong~ ============== ==============

Hello MarshMonster, thanks 4 the response. I can't tell u yet about re-plugging the sensor right after starting, (I'll do that in the morning). As for plugging back in after operating temp is reached, it instantly starts a rough erratic idle and throttle response is alot like a carburated engine with a manual choke too closed down for a hot engine, (bucking & stumbling & surging). As for defining "Weird Idle", the rpms are all over the place dropping down to 200 & surging up to 1200-1500 rpm. Kind of like "A-WOOMP-A-WOOMP-A-WOOMP-A." Sounds you would normally hear from a 8-9 second drag engine, not the family Suburban. Also I have noticed during these times of erratic idle, a very rich smell. I will try what you have suggested in the A.M. I'm following your line of thinking on this, it sounds like it makes plenty of sense to me. I'm a fish out of water in a way when it comes to factory computer controlled / managed engines. My background experience is centered around drag engine assembly, tuning and driving. I lost my personal, "He can fix anything!" friend & guru some years ago. This is the first time I've had a problem I couldn't manage on my own. Again thanks, I'll post tomorrow. NHRA

Hey Steve, Thanks for the come-back. I've only replaced the one that is near the thermostat, the temp. sensor. The other one, located on the driver's side of the engine block, ( gauge / lights), I HAVE NOT replaced. What-cha-thinkin? Thanks again NHRA

I've heard of this "trick" with LS1 motors.

I believe by removing the IAT sensor, the computer finds that "fault" and goes into open loop (and failsafe) mode. In open loop, the computer uses it's pre-programmed/hard-programemd "maps" to run the engine. These maps are usually very conservative in timing and power output.

Is that 100/110 temp on the in-dash temp sensor?

My '04 truck goes closed loop at 132 or 133 degrees.(I monitor this with tuning software). It's indicated on the dash as about 160 degrees, so the in-dash guage is not completely accurate.

It sounds like it's a problem with the car going closed-loop.

Let me look into it further... I want to say this indicates a MAP or MAF issue.

In LS1's - the IAT disconnect trick mainly comes into play on cars that have been recammed with LARGE duration cams, and their computers have not been reprogrammed for close-loop operation yet. (MAP or MAF adjust- ments need to be made).

To make sure it's not a IAT sensor problem, you can temporarily replace the IAT sensor with a standard resistor from Radio Shack (for like $69 cents). I don't have an exact resistance handy... if you need it, I can probably look it up.

-marc

I'd say the coolant temp sensor is stuck in the cold side:

When you connect it in the morning, the engine gets the extra fuel it needs, during the rest of the day it gets too much fuel.

Then in the morning, when it's disconnected, you don't get enough fuel. Because it being disconnected shows as no signal at all.

Refinish King

. . =3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D Aug 4, 2004, 3:43am (CDT+1) From: snipped-for-privacy@nospam.earthlink.net (NHRA)

Hello MarshMonster, thanks 4 the response. I can't tell u yet about re-plugging the sensor right after starting, (I'll do that in the morning).

As for plugging back in after operating temp is reached, it instantly starts a rough erratic idle and throttle response is alot like a carburated engine with a manual choke too closed down for a hot engine, (bucking & stumbling & surging).

As for defining "Weird Idle", the rpms are all over the place dropping down to 200 & surging up to 1200-1500 rpm. Kind of like "A-WOOMP-A-WOOMP-A-WOOMP-A." Sounds you would normally hear from a 8-9 second drag engine, not the family Suburban.

Also I have noticed during these times of erratic idle, a very rich smell.

I will try what you have suggested in the A.M. I'm following your line of thinking on this, it sounds like it makes plenty of sense to me. =A0 I'm a fish out of water in a way when it comes to factory computer controlled / managed engines. My background experience is centered around drag engine assembly, tuning and driving. I lost my personal, "He can fix anything!" friend & guru some years ago.

This is the first time I've had a problem I couldn't manage on my own.

Again thanks, I'll post tomorrow. NHRA =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D NHRA, posts the results and let us know.

I would suggest you gather up ALL the bits and pieces of good info you've posted so far in the thread...and get it all on one easily read page. We've got a few more clues out yer last replies. Then add the words UPDATE to yer header to draw some attention.

(it's always easier to have all the symptoms, conditions, and data on one page)

It sure would be nice to know what codes might be hang'n around inside that black box.

With you in the Race buisness.... you must have a contact or two that has access to a Scanner.??

I would suggest that after you do those little quick tests, that you pull the Battery Negative cable off and clear the codes out. Then drive the truck all day doing whatever it is that you Drag Racers do in real life....then get you a buddy with a scanner to LOOK AT THE DATA and pull codes on it.

It's not absolutely necessary, but would shure save some guess work toward getting the symptom to go away.

Do what you can and get it on the screen, then wait for Doc, Ian, or Bret to chime in with some more input. They're already reading the thread....so get those codes if you can, exercise your patients and do what the Regs ask....and you'll get er fixed in no time.

SIDE NOTE: off topic

One of the guys I work with runs in the "Outlaw" devision. He's running low 4's in the 1/8th.

NOX rules!

hopefully helpfull

~:~ Marsh ~ puts the jug of shroom juice back in the fridge.....thinks it's a Crown&Coke sort of night tonight~ =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

OK, I'm lost in the alphebet soup. How about someone posting a glossary. I know what MAP is, but what's MAF, IAT, and any other abbrieviations re sensor used in late GM apps? Dan

I was too, for a long time... ;)

Here's the common ones...

MAP - Manifold Absolute Pressure MAF - Mass Air Flow IAT - Intake Air Temp ECT - Engine Coolant Temp TPS - Throttle Position Sensor EGR - Exhaust Gas Recirculation

ECU/VCM - Engine Control Unit / Vehicle Control Module - "the computer".

Open loop is when the engine is cold, and the computer is telling the motor how to run from some basis maps or programs that part of the basic operating system of the computer.

Closed loop is when the computer turns on the 02 (and other?) sensors, and uses the data pulled from them, to make decisions on how the engine should be managed.

-marc