Hi. Acceleration "choked" when floored from stop, sometimes at 60km/h

[q45fix]

Why is acceleration "choked" when I floor the gas from stop ? Is my Q supposed to spin the tires on pavement (from stop) when i floor it (with traction control turned off)?It also feels like the engine is "choked" when going around 60 km/h and then pressing the gas pedal about 1/2 to 3/4 way down; the "choking" goes away when the gas is released to about 1/4, then once the car starts accelerating (once released to 1/4) i can press the gas pedal to 3/4 again and it accelerates well.

I have read some threads about this but still i'm not sure what's the issue. Someone mentioned Camshaft angle sensor(s) others mentioned cruise control ...????zerothread?id=152331http://forums.nicoc ... ?id=230117

1994 Q45280000 kmFuel pressure (regulator vacuum hose connected): 33 psiFuel pressure (regulator vacuum hose disconnected): 43 psiCylinder pressure: 165 - 178 psiInjector resistance: within spec.Recently passed emissions in Ontario

[maxnix]

With no maintenance history, impossible to guide you.

Please post such relelvant to fuel and intake systems.

Did you measure actual fuel pressure with a T after the filter?

[q45fix]

Maintenance as per factory shop manual "severe duty" cycle for the past 100000 km dealer serviced before me.

engine oil: RedLine (10w30 summer, 5w30 winter) alwaystransmission: flashed once a year (around 20000 km)All other filters and consumables changed as per shop manual (as mentioned above)

Yes fuel pressure was measured after the fuel filter.

[maxnix]

So no BG44K or Techron?

No fuel filter within last year? Air filter?

By ATF "flush", what exactly do you meant?

When were NGK PF5RG-11 plugs last replaced?

[Infinitiguy19]

Did you check and clean Maf and connector, TB and run AT check?

[Q45tech]

Ideally a dyno test would show engine transmission, tire condition, but a stop watch will do for basic diagnosis.

Acceleration requires a proper ignition advance curve and a proper setting at idle [15 BTDC].........................what are conditions of knock sensors?

The engine supplies adequate torque but really doesn't accelerate smartly until 3500 rpms or 25 mph by design.

Spinning tires is wasteful inthat no acceleration occurs until they stop spinning.

[maxnix]

Adequately sticky tires will not spin. Only billiard ball hard, non OEM specified ones will.

[Q45tech]

Because the rear torque peak is 245 lb/ft x 9.35 [gear + dif multiplication] = 2290 lb/ft divided among rear tires trying to accelerate 4300 lb.2290/4300 = 0.53G maximal acceleration in 1st gear....................PROPER tires can usually accomidate 0.95G before spinning except that the VLSD may take more than 1 turn to lockup or may be defective period and allow a tire ton begin spinning.

Another way to look at it how much weight vs torque is on each rear tire.Statically it starts at 45% but increases as the body [CG] shifts. Reverse engineering by the amount of rear body squat 2.5 " = < 310 lbs added to tire [4300 x 0.45 = /2 = 967 + <310 or < 1277 pounds on tire at maximal acceleration with 1145 lb/ft................no spin unless tire has less than 0.89G of friction.

Obviously tire friction decreases as tire or road gets colder. Damp roads may be only 0.55G with rain lower and snow around 0.2G.

In 2nd gear the peak acceleration is around 0.3G

[goody90q45]

It also feels like the engine is "choked" when going around 60 km/h and then pressing the gas pedal about 1/2 to 3/4 way down; the "choking" goes away when the gas is released to about 1/4, then once the car starts accelerating (once released to 1/4) i can press the gas pedal to 3/4 again and it accelerates well.

It sounds like you've done your homework Ahmad and kept up on the maintenance and I'm guessing that if you had a CEL, a miss at idle, or had neglected to change the fuel filter you would have said something in your original post. Also, if you heard a swarm of bees in your trunk (dying fuel pump) you would have asked for a good exterminator in the Ontario area.

With that said I'm going to take a stab at it and say that your fuel pump control unit (FPCU) is dying and if that's the case it's sending the wrong current to the fuel pump and killing it too. Assuming you're not hearing the swarm of bees too loudly there's a simple test that takes the FPCU out of the loop, jumpering the fuel pump directly to ground and running it on high speed only. If the jumper to ground solves the problem it's a bad FPCU.

The pic shows the FPCU connector and the FPCU in the background. It is mounted on the underside of the rear parcel deck. Access is through the rear trim panel in the trunk. If you already have the trunk trim out you've got the hard work done. Run a jumper wire (of the same size as the wire you're grounding) from the lower left pin in the connector to an unpainted grounding point on the chassis. Use alligator clips or equivalent but make sure you have a secure contact. If your Q accelerates well with the jumper it points to a bad FPCU. Let us know what you find.

[maxnix]

I don't know what disconnecting the vacuum hose from the fuel pressure regualtor will do, but he reports 43 psi. So I did not think FPCU could be the culprit unless it is intermittent.

[goody90q45]

I'm thinking it's a measurement at idle only and we don't really know what's going on (maintaining 43psi?) under load (acceleration). The jumper test is easy to do (he's an engineer) and rules out the FPCU pretty quickly.

[maxnix]

OK, I thought if the pump produced 43 psi, chances were good it would sustain it. But, I guess the solder could melt under load.

[q45fix]

Gentlemen, thank you for all intelligent, logical and insightful answers. I'm reevaluating the possibilities.

Additionally one other (perhaps unrelated) symptom is that:Occasionally when the car is restarted (after it's been fully wormed up) the RPM will go to and stay at around ~900 in neutral (normally RPM holds extremely steady at around 500; neutral or drive, no matter what's on: AC, lights, etc...) This is corrected after 1 or 2 attempts of shutting the engine off, waiting around 10-20 seconds and restarting.When the "900 RPM symptom" is triggered; in drive (with break on and stopped) the RPM stays at steady 600 and (in neutral) the car pulls forward significantly stronger when break is removed (gas not applied) VS. when the RPM in neutral is at 500.

[Q45tech]

Easy to use an ammeter to measure the current flowing thru the pump fuse. Substitute ammeter for fuse.

No current no pump rotation.

Good method because you can see oump steps as current increases with load and rpm of engine [plus the initial cold crank wide open pump time].

[maxnix]

For that symptom, I would say it is time to clean your IAC valve as it seems to be sticking.

We really need you to answer all the maintenance questions to eliminate some of the subsystems.