1991 Q45 Stalls

[QShip]

Geeeeezzeeee!!!

The car stalls when it's not thoroughly warmed up. Even when the temp gauge has moved from the bottom position, it will stall.

I have to drive her non stop for a good 10 minutes before she acts normal. It only happens when I come to a complete stop.

I have a new fuel pump, fuel filter and I visually checked the inside of the controller and it looks ok. New plugs as of last summer. Also rail flush, transmission flush with pan drop and new O2 sensors.

On a few occasions, the LOW BATTERY CHARGE warning light comes on. I recall someone saying that their leaking valve covers caused their alternator to go bad. Mine are leaking. I will check the alternator this evening.

[Q45tech]

The alternator is like a 3 phase motor.........2/3 of it can fail and still put out some recharge current [or not].

The fuel pump, ignition, injectors etc power consummed to run the motor approaches 10 amperes.Stalling during warm up [prior to coolant reaching 176F] usually means dirty air passages and a resulting stress on ecu to try to maintain correct warm up speed.

When you first crank the Q what is the idle speed in park AC off in 30 second increments........1500>1200>1100?

[QShip]

Thanks for your reply Dennis.

I cleaned the MAF, (used the electrical cleaner), and the throttle body is clean.

I will check the idle speeds when I leave work.

[Q45tech]

Wiggle the coolant temp sensor connector that feeds ecu [2 wire not single that feeds the gauge......corrosion in connector?Takes hours and hours to check all sensors without a Consult.......other than wiggle test.

Turning the IAC Air bypass screw all the way in then out exactly 3 turns tells you if there is an air flow restriction in TB or IAC.

[SFBayQ45]

Geeeeezzeeee!!!

The car stalls when it's not thoroughly warmed up. Even when the temp gauge has moved from the bottom position, it will stall.

I have to drive her non stop for a good 10 minutes before she acts normal. It only happens when I come to a complete stop.

I have a similar problem. The car stops upon a complete stop, especially in the early morning hours when the car is not completely warm. There aren't any problems once the engine is warmed up after 20 minutes of city driving. I also now noticed that it now stalls when I turn the steering wheel all the way to the left or right (for U-turns) on cold morning hours. Is this the identical problem you have QShip? If so, I would love to know the fix...

[911/Q45]

At lock, the power steering pump puts an increased load on the engine. So, if it's on the verge of stalling-it stalls. Steering really doesn't have a direct effect.

[QShip]

When I first cranked the Q, it was at 1500(a split second)>1200(about a minute)>1100(30 seconds)>1000(30 seconds)>750 rpm.

I read 12 volts before I cranked the Q and 14 volts at the battery terminals while the Q was running.

I have not yet wiggled the coolant temp sensor connector.

[Q45tech]

I suspect that the IAC may be dirty or not functioning.

The idle speed is maintain bypass air flowing under the TB plate about 80%, then 10% flow around IAC, and 10% from IAC solenoid pulsing open from ecu commands.In other words [at idle] 500-550 rpm from under TB air, 600 rpms from IAC bypass and the final 650-700 rpm from IAC.

The warm up idle compenstation is most done by the hot wax cam cracking the throttle open a tiny bit [to take some of the load off ecu and IAC] the warm up should be complete in 10 minutes unless extremely cold [below 30F crank].

Sometimes an old or bad thermostat will leak coolant around its edges and delay warm up or never let it occur in cold weather.

The Consult shows the real rpm, coolant temp, and the IAC duty cycle = 10%-15% at 176F in park at 650-700 rpm.

When AC or PS is under load the IAC duty cycle goes up ~~ 30%, when the engine speed is higher [say 60 mph] the IAC is up to 60-70% so that when you decelerate the rpm drops in steps to avoid an instantant off throttle stall.

[QShip]

Thanks Dennis.

I will have the IAC checked.

[psychic_mechanic]

My 94 has been stumbling a little bit while it's cold, but not when idling. It shudders a bit at 70-80 MPH at part throttle. It clears up at WOT. If I pull to a stop it will idle rough for a few seconds and then even out. It hasn't stalled or come close to it though.

As soon as it is all warmed up and reading inputs off al the sensors, it evens out and is as smooth as ever. I guess I need to check all the connections.

[Q45tech]

Check for knock sensor codes also.

[Q45tech]

Speaking of KS we must have replaced half a dozen this year so far on 97-99Q.Cracked and corroded.......such is the heat from ATL traffic.......when they get above 100k.

[SFBayQ45]

I suspect that the IAC may be dirty or not functioning.

The idle speed is maintain bypass air flowing under the TB plate about 80%, then 10% flow around IAC, and 10% from IAC solenoid pulsing open from ecu commands.In other words [at idle] 500-550 rpm from under TB air, 600 rpms from IAC bypass and the final 650-700 rpm from IAC.

OK. Just got back from the shop and my only trusted source now confirms your initial suspition...it's the Idle Air Control (IAC) valve. My next question is: Can this be a DIY job? How involved is it to "clean" the IAC valve? Am I better off buying a new one and replace? I haven't called Scottsdale yet, but does any know a ball park figure for one of these things? My car just stalled next to an old Jag today...that's terrible considering how unreliable Jags are...

Help needed fellas...

[Q45tech]

The critical thing is to have a new IAC gasket on hand [just like having a new MAF gasket when you remove that].

Hard to get to the IAC input output hoses and their clamps. The hoses will be brittle and moving them will cause cracks and leaks.

So have the hoses ready and on hand.

Either way with a new or your used cleaned up old unit.

[QShip]

If the IAC is malfunctioning, will it throw a code that can be pulled from doing the self diagnosis with the ecu?

[Q45tech]

NO! And what pray tell code number would that be on a 90-95 Q?

Even the Consult has no method other than displaying the commanded IAC duty cycle and engine speed. What's commanded may have no bearing on actual flow......no method to monitor air flow and proportion it among what is flowing into engine under the TB plate and how much is bypassing thru IAC to plenum.

Dirty clogged up [sometimes to the point where the IAC piston is frozen in place] or downstream or upstream from piston the passage or a hoses is clogged.

The 90-95 Q ecu diagnosis capabilty is dumb as a rock. It really only reports when something is totally dead or so far out of normal operation that running is in immenent danger of ceasing.

A few things try to infer what the problem COULD be but they are too broad and SOMETIMES confusing to the layman:Injector leak DOESN'T mean a leak just that the engine is running rich in one bank part of the time [a misfire].

[Q45tech]

If the duty cycle is above 15% at warm idle in park [no AC no headlights no power steering load] you immediately know something is wrong.

The ecu will command the IAC to open up to 60% [which should result in a 1300-1500 rpm idle [without other loads].....seeing a 25-30% or 40% [at 650-700 rpm] tells you things are very very dirty.

[SFBayQ45]

Q45tech - If the IAC is defective or dirty, could this be the reason why I'm also feeling a slight vibration in the engine (that travels up to the steering column) whenever I step on the gas? One source told me that it feels more like a starvation of fuel or something, rather than a tell tail sign of bad injectors. As a reminder, I've already replaced two new O2 sensors in the front, fuel pump & module so I'm thinking it's not a gas flow problem, right? His explanation is that an injector is like a light bulb, it either goes bad or it still stays lit. In his opinion, it can't be the injectors because my Q idles smoothly and only vibrates ever so slightly upon stepping on the gas pedal. If the IAC controls the air coming in and it is not at 100%, then could this be the culprit to his "starvation" theory?

Qship - Have you already replaced your IAC or clean it at least. Do you have any suggestions of the procedure. Any pics to show me the steps along the way?

Any suggestion/assistance would be greatly appreciate fellas...

[QShip]

I have not cleaned the IAC yet. However, I did find some posts that tell how to clean it.

I recall Q45Tech says to order a new gasket for the IAC and the hoses will probably need to be replaced. He also mentioned to make sure you clean it as thoroughly as possible.

Do a search for 'cleaning IAC' and see what comes up.

[Q45tech]

A brand new IAC only flows 10-15% of the air required to idle at 700 rpm after warm up. The other 85% passes under the slightly opened TB plate.

In the FSM there is a test where you disconnect the IAC electric connector and the rpm drop by 100 rpm or so.........as all the air flow is now just that amount from the bypass screw setting.

If you thiink about it each 10% increment [time open vs time closed] of IAC solenoid duty cycle is about 100 rpm.

The system tops out at around 1600-1750 rpm.

Cruising at 60-70 mph the IAC duty cycle increases to try to almost match the rpm in 4th gear, so that when you suddenly let off throttle the engine doesn't die. It just drops 300-400 rpm initially, then drops in increments [downward steps] to slow the speed.

Not having the IAC clean and set correctly makes for some weird things at throttle closes at speed besides the usual idle problems.

The higher the duty cycle used to satisfy idle air the less dynamic range the system has to deal with highway changes.

[squeefoo]

Q45tech: "wierd things at throttle closings at speed"

Could you offer more detail on this statement? Mine surges when I lift suddenly above 50 mph., and will die when I stop suddenly when cold (very rarely after running for 1 - 2 mins.) other than that it's great! Last summer I cleaned the IAC 'til hell wouldn't have it, but no adjustment. Thanx

[Q45tech]

Plug your Consult in and set the duty cycle.........or turn the bypass screw all the way in, then out 3 turns.If the hot idle is not perfect then you still have dirt problems somewhere.

Weird things : not a smooth even step down in rpm [like stair steps]............don't confuse with locking unlocking of TC.