AAC test for Q45

[firstq]

I am trying to diagnose 2 issues with my 92Q

1. The car gets some minor vibration at idle - it is enough to feel in the driver seat, driver side door, and the steering - it would be fair to say it is not 100% quiet - I am not sure what to expect from a 10 year old car, but nevertheless some vibration is there (this is when the car is stopped and idling with gear in the "D" position - the vibration lessens if shifted to "P")

The fuel rail was flushed very recently with 2 cans of berryman's B-12 pressurized can, and it helped some - the engine is a lot smoother at startup (in the morning) but the idle vibration is still there.

I am not sure if the idle is set appropriately, I have read Dennis stating that the idle speed of 600 to 650 to 700 makes a lot of diference in vibration - and I having no consult have no way to distinguish the +-100 rpm by looking at the dashboard meter. I think the RPM needle stays above the mid notch (but below the 3/4 mark).

I have not done a power balance test, perhaps hesitant to take it to the dealer at this time.

Observation: When I disconnect the AAC connector, the speed does not drop at all - I have checked the AAC resistance and the connector voltage and they both checked out fine.

Is this a problem? (that disconnecting AAC is not making any difference on idle RPM - the manual says it should drop, right?)

2: The engine is fairly sluggish and rough in the morning- only when the engine temp needle reaches the mid level on the engine temp meter - does the engine runs smoothly with all the power - otherewise in the morning, the sound from the engine is like driving a tank ( for the first minute or so after starting in the morning - I am in AZ, so it is not freezing outside, even these days, but still I have oberved that in the sun, the engine is not that noisy in the mornings) - this smooths out after a minute or so.

Observation: The brake pedal is sluggish in the morning (requires more force - right after starting up) but smooths out like gliding [much softer] when the engine warms up - could it be a vacuum leak only affecting the cold temp pathway?

Also, whiel starting the enging, the first second or so in the morning, something makes a splutterring sound as if something is not oiled or something (having difficulty to turn) and then after the first second, it smooths out. All the belts were changed, may be one of pulleys are acting up, what do you guys think?

Any pointers...thanks

[Q45tech]

The IAC is designed to operate at a 10-15% duty cycle [adding 10% more air than flows under the closed throttle body] at 176F coolant temp in park no ACIf the rpm doesn't good down when IAC is electrically disconnected it is not set correctly or is dirty or malfunctioning!

As the engine ages the amount of dirt [PVC] increases speeding up the dirtying of the throttle body at 240k mine gets cleaned every 2-3 months whereas a brand new engine might survive 2 years when new...........progressively increases in blowby.

But then again my standards of smothness are probably 2-3 times higher than the average owner.Screw the IAC manual bypass adjustment all the way in then back out 3.5 turns and clean [keep cleaning] the system still the rpms come up to correct............works every time! Except when IAC solenoid /plunger is stuck and won't let the computer add air.

"perhaps hesitant to take it to the dealer at this time."......Never heard of "FEAR of Dealer".......is this related to fear of doctors that they will tell you .........you are dying.

If all the cylinders are putting out the same amount of power the idle will be PERFECT but it requires 650 rpm in drive, in park 750 is more normal.

Youi can't trust the tachometer to delineate between 650-675 or even 700 rpm.

[firstq]

Dennis, thanks for your input.

1. I cleaned the TB may be 6 weeks ago (the TB was in my hands - was doing a plenum job - most of you guys helped me out there as well). So I think, the TB is fairly clean (per my standards of "cleanliness")

2. I also cleaned the AAC as it was disconnected and was in my hands as well - the usual intake system cleaner solution with a tooth brush.

So both TB and AAC should be clean (as clean as I can get it myself).

In your response : "...Screw the IAC manual bypass adjustment all the way in then back out 3.5 turns and clean [keep cleaning] the system still the rpms come up to correct..."

Tell me what is 3.5 turns - are you counting a full 360 degree as one turn or just 180 degrees (just want to be sure, I know it is 360 degrees). The reason I ask is because if I do full 360 3.5 turns the idle stays way up near the 900-1K range, I have to screw it in atleast 1 full 360 turn for it to come back to the "visual" 650-700 RPM range - this gives me around 2.5 full 360 degree turns to bring it down to the 650-700 range - can this be a problem?

I am still not sure if I have a vacuum leak somewhere - the brake issue I listed in my original email still perplexes me, is that normal?

Thanks

How much drop is expected when disconnecting the AAC electrically? 50,150,200 rpms? I hear no change in the engine speed and visually no change on the dashboard rpm needle

[Q45tech]

Thr IAC may not be working. [from your cleaning handling or before].Each 360 degree turn should have about 50 rpm variance since 90-100% IAC yields no more than 1300 rpm in neutral/park.All the above applies to a fully warm engine [15 minutes of running].

A perfectly set IAC will hold the rpms constant within 25-50 when the AC is turned on, the steering moved full to lock, all the accessories load the alternator, etc.............

What does the idle vacuum read [a tee installed in the fuel pressure regulator vacuum line] {= > 18"HG?}.

What is the real initial timing 15.000000 degrees BTDC?

Without a Consult and someone who understand how to really use it and someone who knows the engine in their sleep you are in trouble.

Remember the screw is an air bypass around the electrical solenoid plunger opening controlled by ecu.

[firstq]

Dennis:

- The timing is set at 15 degrees (per the tech who worked on the timing guides)

- I have absolutely no clue what is the vacuum reading - any easier way to confirm if it is ok or not before actually going ahead and getting the exact reading?

I tried pinching the blowby hose to see if there is any other vacuum leak, but no change in the RPM

I might try the choke cleaner/water mist spary technique to locate any leaks.

Thanks

[Q45tech]

Vacuum guages are cheap at Sears and to compare to my numbers always tee between the fuel pressure regulator and plenum hose vacuum feed.

Most of the time [after a cleaning] the injectors/back of intake valves, plugs are dirty.........why a rail flush is needed every year or so even with 44k every 90 days.

All I can say after 13 years [241k] with the same injectors it is possible to have absolutely no RPM variance in a power balance which means the engine is as smooth as new at idle....[ignoring the Sounds of injector wear and valves [HLA].

Members can chose any level of vibration they can stand. It is only a function of money and time.

Measure the idle vacuum 18"? no variance not even 0.5 psi [=14" WC] is significant .

Many techs read the 15 degrees from the Consult [not knowing that the 15 degrees is the set point meaning the CONSULT assumes that the real timing is set absolutely at 15 degrees......only a variable timing light and the crank marks tell you for sure ..........but the timing chain can stretch 1,2,3,4 degrees and the crankangle sensor is fed from the drivers exhaust cam so crank vs cam timing can be get off with miles....usually not significant as the VVT retard at idle covers up any instablity..........soft acceleration 1000>1500>2000 rpm is the telltale sign with a much better than normal 5500>6000>6500>7000 acceleration...........old engines perform better near redline due to chain stretch {exhaust cam retard vs crank acts like a bigger [more duration] exhaust cam} with more overlap at idle allowing more reversion and intake charge contamination so the injectors better be perfect on 120k+ engines.

Once you get it perfect it is easy to learn to feel the start of vibration telling you it is time to reclean...........a 25 rpm drop will let you know [feel] it in the steeering wheel when the car drops to idle!

It amazes me what owners put up with! Every qiver has a cause!

[Q45tech]

Which is why I get so upset with K&N owners, the engine creates enough dirt [PCV and EGR, idling, etc] that you don't have to add to it by a filter problem.

A 120k requires twice the maintenance of a <60k engine and a 180-240k requires double that..............3-4 times more frequent than the factory owners manual ever contemplated an owner would drive.

When I tell my Japan Nissan friends about 241k they just go .......Wow Unbelievable ["you Americans drive too much'] ............our 448k 90Q keeps chugging along after being rebuilt a few times.

[firstq]

Okay - so my next work item is to find a vacuum gauge and tee it at the fuel pressure regulator vaccum feed. I am there so far, however; where is the variance test coming from? Are you talking about variance b/w reading at idle and at WOT? or the variance b/w the 2 fuel pressure regualtors?

Thanks

[Q45tech]

No vacuum variance as the engine fires all 8 cylinders, the vacuum is steady very steady. Each cylinder is the exact same size so the inward flow should be exactly the same [if the rpm is steady].

They make an electronic vacuum gauge which can be attached to an oscilloscope so that you can see the individual ramp ups and falls [Gaussian curves] of each cylinders vacuum same with monitoring the MAF voltage it will vary minutely with each suck [not really a suck as the atmospheric pressure just forces air into each] by each cylinder.

[Q45tech]

The Sears vacuum gauge is 0-30" HG and doesn't offer the resolution to view what's happening at WOT ~~~1"HG or less.

[firstq]

1. Okay, I got the Sears Vacuum gauge ($20).2. Drove the car for good 30 minutes3. Used the "t" supplied to connect the Gauge.4. started the egine - the fuel pressure regulator the vacuum is steady at 19". 5. Where do we go from here?

Thanks

[reggiegsd]

I'm always amazed that 1" - 2" HG diff pres will give the flow rate an engine needs at WOT. I wonder what the pressure is at the intake valve?

[Q45tech]

The pressure assumming you are not at a tuned point [where the reflected wave reinforces the incoming atmospheric pressures] is 14.7 LESS the restriction or 14.2 psi at the absolute instantaneous peak, but the restriction starts out high as the valve just opens reaches a peak then declines......so the total or average air flow......guassian/Normal curve.The ring sealing is not really good enough to suck in air, air just flows in to reflect the void as the piston drops! This is the vacuum you read across the intake restriction.

The vacuum is [shows inversely] a direct relationship with the volumetric efficiency.

When heads are measured on a flow bench the industry standard is 28" WC [how many CFM at 28" WC] which is equal to roughly 1.0 " HG. .....or 1/2 a psi.Years of trial and error have develop this optimum point

If you think about the distribution of the various restrictions: 7.5 outside air to MAF, 5.0 in MAF itself, 2.5 in hose to throttle body we are at 15.0 to input of throttle body total at 6500 rpm! Leaving 13" or so for the TB, plenum, runner, heads and valves.

The main problem is at low rpms the cam opens the valves for 248 degrees [seat to seat] which means that for 248-180= 68~~48 degrees {VVT} [really about half this as the valves needs time to move off seat] air is being forced back out as the piston rises until the rpm increase enough to get the air speed up enough so that this reversion becomes a packing [supercharger effect [trying to walk against the wind] at 3,000 rpm or so!

Life in a street engine is always a compromise against idle low speed operation vs high rpm peak power and the Q [2] 1.5 inch diameter intake valves are on the big side [1.76x2=3.53 square inches -----much bigger than a single 2.02" [3.17 square inches]racing valve for a 350 Chevy which makes 400-450 HP].http://www.edelbrock.com/autom....htmlh ... rules.html

The Q engine is optimized for PEAK TORQUE [maximum numeric value] the area under the 3,000-4,500 rpm band is optimized.

A maximum HP optimization would be at 7,300 rpm which means the peak torque [same amount would produced at 5,500 rpm] would yield around 460 HP give or take a few.......but the engine would need to idle at 600+1500[amount torque rpm was raised]= 2100 rpm..............you could lower this [to 1200] by raising the compression by 4-5 points[say 15:1] and burning Methanol........notice that the Indy 4.5 Infiniti motor only made 375 lb/ft at best just 38 or 11% more than a Q with JWT ecu.

Physics does interfear with wishes!

[Q45tech]

The coefficients [bore raised to the 1.75 power] vs stroke raised to the 0.587 power] in a classic maximization equation show you which is more important in getting the most HP from a cubic inch.....the area of the piston vs the stroke................stroking engines beyond the normal point does add power but at a less than optimal rate.........while increasing the stress seriously lowering the peak safe rpm.................were efficiency counts [gas mileage] big bores are it..........there is an optimum size for each engine configuration ............2 with 2.5 liters as the max on a I4, and 4.0 liter with 4.5 as the max for a V8.............see American engines have always traded off efficiency for size and low rpms.

Mercedes [5.0 liter [305 ci] now 5.5 liters [335 ci] has taken a middle ground with 3 valves and less than 350/346 Chevy.

Whereas Audi has used 5 valves to tweek 4.2 liters and Lexus has gone to 4.3 with 4 valves......when they go to 5.0 liters they will be trading off efficiency instead of building a V10 or V12.

[reggiegsd]

So, to summarize, without an extra source of oxygen, we're stuck at 287 hp.

[Q45tech]

No the JWT WOT optimzation reduces the over rich [for cooling] mixture and adds the 3 degree progression of timing advance above 4,000 rpm and the decreased sensitivity of KS yields around 307 HP at 5500, a dip[to 300] then back to 307 HP at 6,000 rpm..............some have reported 315-318 HP with extremely LOUD exhausts and intake tweeks on the dyno......setting timing to 16 BTDC in winter.

With JWT, I think of it as a 300 HP/330lb/ft engine year around assuming you don't use oxygenated fuels then it is a 285 HP engine at best.Maybe 225-250 HP on regular oxygenated with JWT worse than stock because of knock?

Actually if you lean the engine more till it almost explodes you might get to 340HP for a few seconds but the damage might be extensive....................The JWT Nitrous package gets to 360 HP and 395 lb/ft for the 6 runs till the bottle is empty.

[reggiegsd]

Has anyone really pushed this engine for WOT performance? Tuned exhaust header, after market FI with multiple throtle bodies (not turbos or NO2). I suspect it would be a money be damned search for a realitively small increase in WOT performance and a loss of low end performance and drivability.

I bet this engine would be a dream in a 2800 lb roadster.

On a completely different topic. What is the limiting factor for RPM in this engine. On a stock L24 motor (early Zcar) the valve springs alow the valves to float at about 7300 rpm. A natural rev limiter.

[Q45tech]

The long term rev limit [7300] is from piston speed [primarily rod strength and the wrist pin housing on the piston]. Stroke length is the primary controller of peak [redline] rpm. Thus the ratio of rod length to stroke length is the gating factor at determing piston speed-----each piston material has a certain material strength. Unfortunately for most material the dimensional stability gets worse with strength: the piston expand in the bore when warm and stretch above the deck when extremely hot........cast is more stable than forged which is stronger...........unfortunately the engine [piston] must crank cold and warm from -20F to 500F

If your life depended on it I'm sure the springs don't float till 7700. This is because the 1.5 smaller valves weigh less than the single big 1.95- 2.02 used in American V8. Also American valve springs are stiffer thus more brittle.

[sheadee240]

I am planning on cleaning and adjusting my AAC. I was wondering if you could give a little more detail about

"Screw the IAC manual bypass adjustment all the way in then back out 3.5 turns and clean [keep cleaning] the system still the rpms come up to correct............works every time! Except when IAC solenoid /plunger is stuck and won't let the computer add air."

What would you use to clean the AAC. I'm assuming carb clean.

My fiance's '94 has been off the road for 2 weeks now. I suffers from hard starting (crank no start) on occassion. It eventually starts. It also almost stalls at highway speed coming off the gas (intermittently). And also stalled when idling warm on one occassion. It sounds like FPCM AAC and MAF are all possibilies? Any suggestions?

[Q45tech]

Always read the ecu codes first either by ecu led blinks or a Consult.Avail yourself of the information built into ecu for diagnosis!

[sheadee240]

I will do that first thing when I get back to CT on Fri afternoon. There is no check engine light (MIL) but I'll check the codes first thing. All of the drivebility complaints are translated from my fiance (very limitet car knowledge). The only thing I noticed myself was after driving the vehicle completely warm I gave it a few revs in park and noticed that it wanted to stall instead of settling to a smooth idle, but if I just touch the gas it would settle on a smooth idle. Also Ever since I bought the car I have noticed increased gas pedal travel is necassary for drive off, It goes away in about a minute, usually just noticed the first time I engage drive and drive off (It isnt a transmission slip, but rather some type of lack of engine response)

Thanks again

Always read the ecu codes first either by ecu led blinks or a Consult.Avail yourself of the information built into ecu for diagnosis!

[Aussie Q45a]

[QUOTE=firstq]I am trying to diagnose 2 issues with my 92Q

1. The car gets some minor vibration at idle - it is enough to feel in the driver seat, driver side door, and the steering - it would be fair to say it is not 100% quiet - I am not sure what to expect from a 10 year old car, but nevertheless some vibration is there (this is when the car is stopped and idling with gear in the "D" position - the vibration lessens if shifted to "P")

Have a similar vibration in my '90 Q. Local tech reconed it was engine mounts. He put a piece of wood on his floor jack and just took a little weight off the mounts by jacking under the sump slightly.

What do you think Q45tech, is he conning me or what?

[Q45tech]

I change my engine mounts every 120,000 miles that was 1997 and 2002 soon time again at 306,000. The rubber fails.

[Aussie Q45a]

I change my engine mounts every 120,000 miles that was 1997 and 2002 soon time again at 306,000. The rubber fails.

Do you think worn mounts could cause the fine vibration felt in the steering wheel and in your bum through the seat?

[DominickJ30]

Do you think worn mounts could cause the fine vibration felt in the steering wheel and in your bum through the seat?

YES!