91Q - Slow Acceleration

[tl91Q]

Due to a rough-idle I was experiencing in my 91Q, I had the injectors and accompanying harness/hoses replaced last week at the local dealer. The idle is now very smooth, but the car accelerates slower. Even with the rough idle, it was always zippy. Now it feels like it's always working extra hard to get moving. My 0 to 60 MPH time is now about 12 seconds.

Here's some more info on service I've done in the past year:

Replaced Fuel Pump and FilterReplaced Spark PlugsCleaned MAFCleaned MAF ConnectorsRepaired IAC Vaccuum Leak

Any ideas on what could be causing the weak performance?Any recommendations for a good mechanic in Utah?

Thanks,TL

[911/Q45]

Sounds like bad knock sensors, could have gotten disconnected while the under plenum work was happening. You can interogate the ECU to see if there is a code.

[tl91Q]

Thanks much... Does this need to be done with a CONSULT? I guess it's back to the dealer. Is there a place I could purchase a CONSULT for DIY work?

[911/Q45]

You don't need a Consult, the shop manual has the codes and you can just detach the ECU from the passenger footwell and watch the red light flash.

[Q45tech]

Doubt that the knock sensors will reduce [zero/60mph] acceleration by 5 seconds....1 or 2 maybe.

Low fuel flow [measure the pressure during acceleration], measure the TPS voltage during acceleration, measure the ignition advance during acceleration. Mwasure the injector opening time during acceleration.Data log these in 50 rpm increments!

Difficult/impossible to do without Consult.

Now a 2nd gear acceleration [no downshift to 1st might seem like 12 seconds].

[tl91Q]

Thanks. This '91 Q has a '93 TCU, so it does start out in 1st gear (which is usually very quick - nice).

Can the dealer log all of the data items you have mentioned if I tell them to? I get the feeling that they don't know how to think outside the box when diagnosing. Also, what is TPS Voltage?

When I replaced the fuel pump, I did not replace the fuel pump controller - but I have an extra one (out of a '93 Q). Is it worth swapping that out just to see, or should I do all this Consult testing first?

[DAEDALUS]

The dealer can do it and will probably charge 1 hour or more labor. TPS is the throttle position sensor. The voltage tell the ECU where the butterfly plate is. The Consult will not tell them what the fuel pressure is though, it needs to be measured manually. IMO it's easier to check the fuel pressure than to swap the fpcu.

[Q45tech]

You can unplug the connector from the mounted FPCU and plug it into a test one hanging in the trunk . Make SURE you have a good ground wire alligator clipped to the metal case and a ground point for testing.

TPS= throttle position sensor 0.4 volts close going progressively up to > 4.0 volts WOT.

I'm not sure most [any] many technicans routinely do what I described and if they do, they may not know or remember what correct values* are based on rpm ambient temperature, barometer pressure.

* Absolutely correct values for the particular situation are not listed by Nissan [anywhere] they must be calculated for each case especially injector open time and timing advance vs rpm. Most important is the MAF voltage vs rpm.

I like to take 6-8 WOT back to back runs* and data log each portion 600-3500 rpm and 3500 to redline shift and 5500 thru shift to see the transmission shift time and return to 4500 rpm and progression up to 70 mph. This needs a special level SAFE place to conduct the testing.......usually not found near dealerships or the Police.

Pull over between runs print out data segments as approriate, review them mark them up and do it again and again .......then spend a few hours reviewing them.

[tl91Q]

OK. I'll have the dealer run these tests for me. Do you know if and where I could buy my own used CONSULT?Thanks.

[DAEDALUS]

Used they go for over $1000 typically on Ebay, and may or may not come with the right cards for your car. If you really want one check there frequently, or see if any of your local dealers want to get rid of one.