Will92

Hi all:

I'm in Colorado and am curious if 87 octane is causing delayed ignition timing in my 95. If so, I want to switch to 91 octane since I understand that should help with the performance loss experienced at high altitude. I have never used a scanner. Is there a basic unit that folks would recommend that would reveal timing information and do you have to use them "real time" going down the road or while idling or is the information stored in the car's computer such that the scanner "pulls" the information when the car is off. How is timing information even displayed by the scanner and will the instructions with the scanner provide an explanation so I can simply determine if the timing is being delayed or not.

Finally, has anyone heard of adjusting the MAF sensor at high altitudes? A dealer here years ago offered that service after I moved here and I never knew if this was necessary or if the car automatically adjusted the MAF for the higher altitude. Thanks

JvG

Our cars are intended to use premium gasoline.
The timing is advanced to where Nissan intended it to be when you use premium.
The timing is retarded then regular or plus gas is used. Both performance and gas mileage suffer somewhat.
If your knock sensor is defective, the ecu retards timing.

I have never heard of adjusting a maf for high altitude. Altitude compensation should be part of what the ecu does.

lux97Max

iTrader: (24)

Will92

Thanks fellows: Are you both pretty confident that with 87 octane at 5000 ft altitude that my '95 is retarding the timing? If so, I think I will just go ahead and make the switch to the highest octane out here (91) without bothering to attempt a scan. The car definitely runs OK on the 87 (no pinging/knocking) but now I'm curious if acceleration/power will be better if I switch to 91. I always used 93 before I moved to Colorado but started using 87 under the perhaps false assumption it was adequate for our engines at this altitude.

JvG

To the best of my knowlege, octane can be reduced by two points at higher altitudes. So 93 becomes 91.

Our cars were designed for premium, regardless of altitude.

Try a tank or two of premium and see what happens. At worst there will be no difference in mileage or performance. But it is likely that performance will increase. Which is exactly what you want to see at higher altitudes. Your mileage might increase as well. This will offset high fuel costs.

maximaxi

ANot entirely correct. Modern engine management, particularly with a MAF, compensates for the thinner air, so you need the same octane regardless of altitude.

No. The MAF adjusts itself. That's what it does; the MAF tells the computer to adjust the timing.

It's possible that you have a bad MAF and/or knock sensor; they are both common problems in these cars. You can read the codes with the scanner, and it will help you point to the problem. Almost any scanner will work, except the really cheap Chinese ones on eBay.

87 octane is perfectly acceptable at any altitude.

Will92

Thanks: If the MAF sensor or knock sensor were problematic, would the CEL light be on? I don't have a scanner and my favorite Nissan places wants $150 to run the diagnostic.

I'm glad to hear what you said about 87 octane. It says it is OK especially at high altitude in the owner's manual but still I was a bit worried after reading the dozens of threads here that maybe it wasn't the best octane to use. I'm definitely going to give 91 a try here (the highest Colorado offers) and see if my acceleration/performance is a bit better. It is definitely OK now, but not what it used to be when I lived out east at much lower altitude.

maximaxi

A bad MAF will turn on the CEL

A bad knock sensor will not necessarily turn on the CEL, but you will get code P0325 when you plug in the scanner. If the CEL isn't on, and you get P0325 and no other codes, you need a knock sensor. It's about $40 on Rock Auto (Hitachi KNS0001)

You can buy a basic scanner for much less than $150. You can probably get one for $50-70. Actron, Autel, Innova/Equus are all good brands.

JvG

Maximaxi, what I meant about octane reduction at high altitudes was that the oil companies already sell lower octane gas at higher altitudes. So they sell 93 octane at sea level, and 91 octane at high altitudes. They call both premium gas.


Agreed on the scanners you mentioned. Also on the knock sensor information.

Will92

MaxiMaxi:

Why did you mention I might have a bad MAF and/or knock sensor? I mentioned one Nissan dealer offered to adjust the MAF sensor for the altitude here in Colorado after I moved here from further east. There wasn't any real problem in my judgement - just a perceived loss of power in general at this altitude. Should I buy a scanner/have it checked?

maximaxi

Will:

There is no way to 'adjust' the MAF. If it's bad, you have to replace it. The MAF, along with the knock sensor, will tell the computer to make any adjustment necessary. The MAF tells the computer how much air is coming in, and the knock sensor tells it when knock occurs, so it can dial back the ignition timing a bit.

I mentioned you may have a bad MAF and/or knock sensor because something isn't working right if you feel a loss of power. If either or both are bad, they need to be replaced. If there is a perceived loss of power, it's because the MAF isn't properly measuring the air coming in, the knock sensor may be not be doing its job, and/or a possible vacuum leak after the MAF (allowing unmetered air to enter the engine and throwing off the air/fuel ratio).

Also, if the knock sensor is dead, premium gas won't help, because it often goes into limp mode and the computer assumes 87 octane.

Is your CEL on?

Yes, definitely buy a scanner!

Will92

Thanks. But the power loss can be real and not due to a defective MAF sensor, right? I was reading that you can have a real loss in hp as altitude increases.

asand1

iTrader: (1)

Less atmospheric pressure means the ECU injects less fuel to keep the ratio correct. Less air/fuel means less power, and boost is the only solution.

I bought a $10 ELM Bluetooth scanner that works great with my Droid.

Look in the stickies here for the diagnostic procedure, you can look up codes yourself with a screwdriver for free.

We have 10:1 compression, and I measured 42-45* spark advance in my car, use premium.

Your problem is likely just high altitude and nothing will hep that. A bad knock sensor will not trigger a check engine light, and the ECU will often set a false KS DTC in the presence of another DTC.

DBear

I second what asand said. My understanding is that you will always lose power at altitude. But if your knock sensor is bad, that will definitely cause a loss of power, although you probably won't notice it until you replace it. And from what I've experienced and read here, when an MAF goes bad, the symptoms are much more pronounced than just generally less power. More like jerking and intermittent complete losses of power.

LOL at the dealer offering to adjust the MAF. You could do that with carburetors that had automatic chokes, but there's nothing to adjust on the MAF. My guess is they routinely tell that to people who move there from the flat lands...

maximaxi

One more thing: What is your gas mileage now? How does it compare to before?

If your car is OK, and it's just making less power because of the altitude, you will get better mpg than at sea level. OTOH, if it's the same or worse, then it could indicate a problem. The reason for this is because the engine uses less fuel along with the lesser air, in order to keep the same air/fuel ratio.

But of course, after you run the scanner on it you can get a better idea of what's going on.

Will92

Thanks fellows. I believe the gas mileage is pretty comparable to what is was before. I have not kept good records on mpg and just go more by how many miles I get to a tank more than anything else. The car runs very smoothly so I truly believe any power loss is due to the elevation here.

Asand1... when you measured the spark advance, was that with premium? What should the value be when the timing is not advanced or delayed, zero? I considering buying a scanner to learn what amount my delay my engine is operating at before I switch to premium. (Just the curious side of me!). Also, what did you mean about looking up codes "with a screwdriver"??

asand1

iTrader: (1)

I always run premium because I am running the KS resistor mod. I replaced the KS with a resistor so the ECM never sees knock, and always runs full advance for the given engine load and and RPM according to the preset maps. This mod was done due to the frequency of KS failures.

In normal operation with a functioning KS, the ECU will run full advance until the KS detects knock. The ECU will then incrementally retard spark advance until knocking stops. Fuel octane, cooling system condition, elevation, ambient temp, and combustion chamber deposits will all contribute to pre-ignition (knock).

Judging by may car with the KS bypassed, 42 or 45* is full spark advance.

maximaxi

If there's a Napa near you, they have a scanner for $40 on sale this month.