'98 Q Fuel Pressure Question

[duck3986]

I've posted in the last few weeks about my sluggish Q issues on my '98 with 92k miles. The problem is worst at lower rpms, when driving at constant speed and pressing into the accelerator lightly (e.g. going from 40mph to 45mph).

I've even noticed recently that the rpm vacilates, under constant throttle, moving up and down enought to notice slightly on the tach.

This problem comes and goes, with no check engine light which is partly why I haven't done a consult. I figure when I bring it in, the car will run great!

Also, the problem is more noticeable in hot, humid weather.

I replaced the plugs and pcv valves and fuel filter (all oem parts). No help.

Then I did the MAF (expensive) but i figure I'm keeping the car for 3-4 more years, and my '86 Corvette had a similar problem that a new MAF fixed.....and boosted performance as well. The new MAF has seemed to boost acceleration a bit, but the car still falls flat and is sluggish about half of the time.

My question: I'd like to monitor fuel pressure while I'm driving. I think checking it while at idle may not tell the whole story since the car idles perfectly. How do people check fuel pressure while driving?

Also, in my service manual, the California models have a MAP sensor which is affected by barometric pressure. Does anyone know why this is important and what affect barometric pressure has on an engine and it's sensors? Maybe that could clue me in to why my Q runs crappy in hot, humid weather.

Thanks,Don

[DAEDALUS]

Get a fuel injection pressure gauge and T into the rail (do the '98's have a fuel schraeder?). You'll need a length of hose long enough for the gauge to reach the windshield so you can tape it there just high enough for you to see it while driving.

[Q45tech]

Air density changes roughly 3.3% per thousand feet ASL. The ecu needs air temperature and baro pressure to double check the MAF for accuracy [as MAF are relatively in accurate at low flows]

The closer you get the injector pulse just right [to begin with], the less and faster the ultimate zeroing in from O2 sensor data will be!

You can buy electronic 0-100 psi gauge and senders for ~~$100 now.

You have OBD2 so scanner are cheap........lots of useful data

[Q451990]

I've even noticed recently that the rpm vacilates, under constant throttle, moving up and down enought to notice slightly on the tach.

I think you may be on to something on the fuel pressure thing... I noticed a slight surging like you described right before the pump took out the fuel pump controller on Q1. It'll be interesting to see what your fuel pressure tests show.

Heath

[duck3986]

Is there a valve on the fuel rail to connect the pressure test?

Anyone know where/what side it's on in a '98Q?

I'm not sure if the gauge I have will fit. I know it fits GM fuel rail valves.

Don

[DAEDALUS]

You can always buy an adapter.

[greg_atlanta]

... I noticed a slight surging like you described right before the pump took out the fuel pump controller on Q1.

I just replaced the pump on my '92 at T-3 last Monday as a preventative measure (old pump noisy when tank is less than 1/2 full) and I'm also getting the surging thing ocassionally when accelerating. Punch the accelerator and it feels very sluggish and then WHOOSH!

I suspect it's a FPCU problem. I guess the old pump and the old FPCU had things worked out. The new pump and the old FPCU don't get along.

Otherwise the idle is smoother and the car feels much better.

Will watch it for a few weeks and see if it gets better.

[Q45tech]

Obd2 will not light a mil light until things exceed expected values by 50%, yet 10% variance from ideal can affect performance.Obd2 just tries to make sure emissions don't double.

People believe Obd2 is too sensitive yet it is the opposite, when the light comes on something is really bad.

Obd1 is worse it just checks to make sure there is a signal from a sensor ------good or bad it doesn't care.

Fuel pressure ratio vs flow is a square root function:

43.4/34= 1.2765 take square root =1.13 ........13% not 27% so at cruise the O2 can easily compensate for 23 psi instead of 34 psi [spec]...........now acceleration will be seriously affected from not having 43 psi. Why you must measure the rail pressure under WOT to make sure.

That's the way pumps age and fail they reach a threshold beyond which the volume and pressure falls off fast.

[greg_atlanta]

I just replaced the pump on my '92 at T-3 last Monday as a preventative measure (old pump noisy when tank is less than 1/2 full) and I'm also getting the surging thing ocassionally when accelerating.

Well, seems to have disappeared. I cleaned the connectors for the fuel pump, FPCU, and fuel pump relay with tuner/electrical cleaner... that may have helped.

I'll watch it for a few more weeks before I put the trunk back together.

[Jesda]

Where is the fuel pump relay located?

Thanks-Jesda

[DAEDALUS]

Excellent info, with pics.http://www.nissaninfiniticlub....69107

[greg_atlanta]

Relay is behind the passenger side trunk panel. It's green.