1990 240sx Bad mileage and idle.

[jerryk]

I have a 1990 Nissan 240sx hatchback with 183,000 miles. The compression results 155-160 dry range and only 2+ on the wet test. My gas mileage is very erratic and I have conducted numerous tests. Highway at 60MPH the best was 20-23MPG stop and go city 14-16MPG. I do not think this is appropriate mileage for this car. I am on 17” wheels and a web calculator shows a small odometer error 59 miles = 60. The car on a test dyno produces very high levels of hydrocarbons but the rest of the pollutants were within specifications. It has a brand new Cat so the engine is really spilling gas. I have replaced fuel filter, injectors, spark plugs, spark plug wires distributor cap, rotor, temperature coolant and O2 sensor, and corrected ground weakness. The EGR has also been tested. The idle is 750rpm, but it cannot go 30 seconds without a stumble some more violent then others this alternates sometimes with an idle hunt between 650 and 850 rpm. The car is equipped with a wrapped stainless header, random flow cat and a Borla Cat back. It has a K&N filter on the standard intake pipe. I suspect the TPS and the crank angle sensor, how do I test them, or where else should I be looking? I also suspect an overly hot engine compartment because of the header and thinking of making a heat shield. I am getting ahead of myself; I had the idle and mileage problem before the upgrades. Which areas should I look at next and what test can I perform without replacing all fuel and emission components blindly. I want to get this car working correctly and I am investing money and time.

I have my original 240sx, rusted out bottom, parts car with 230k on it and I have swapped the MAF. Many engine builders I have spoke to, thinking of another SOHC rebuilt, say my mileage should be better.

Please do not use abbreviations or fail to describe the wire colors and locations for tests.
Modified by jerryk at 7:00 PM 8/6/2008

[1mansho]

you can try changing out the 02 sensor if it hasnt been changed yet..

[jerryk]

you can try changing out the 02 sensor if it hasnt been changed yet..

Whoops, that was replaced with a new unit.

[1mansho]

oh didnt catch that, how bout your fuel filter? could be pretty clogged up from old age

[aziankingz]

yeah i get crappy gas mileage too.. around the same as you actually..i changed almost everything.. going to change my spark plug wires and see if that helps.. are you misfiring also?

[jerryk]

oh didnt catch that, how bout your fuel filter? could be pretty clogged up from old age

I left that off I will update the above it is new.

[jerryk]

yeah i get crappy gas mileage too.. around the same as you actually..i changed almost everything.. going to change my spark plug wires and see if that helps.. are you misfiring also?

Misfiring, I do not think so, just poor mileage sometimes you can smell the gas, and a bad idle. I have bad experience with cheap spark plug wires get the NGK. The idle hunt was gone for two months but it is back it the summer heat.

[aziankingz]

just ordered ngk wires last night.. where in ny are you from? have you checked for vacuum or exhaust leaks? or use seafoam to clean out the intake manifold and also spray some carb cleaner to clean the iacv.

[jerryk]

just ordered ngk wires last night.. where in ny are you from? have you checked for vacuum or exhaust leaks? or use seafoam to clean out the intake manifold and also spray some carb cleaner to clean the iacv.

,

Brand new exhaust system from header to muffler tip no leaks.

What and where is IACV and how do I clean it. I am sending tons of unburned gas out the tail pipe. I ran carb cleaner no change. Vacuum leaks how do I test for them?

[aziankingz]

the iacv is behind the intake manifold near the firewall of the car on the passenger side..very hard to reach and take off. easiest way is to spray carb cleaner from the hose which connects to the intake. if youre standing in front of the car facing the engine bay, its the bigger hose on the left of the intake.. just loosen the clamp on the hose, turn on the car, slowly take off the hose but at the same time rev the engine so you dont stall your car (it takes patience) or have someone rev the motor for you while spraying into that hose. best way to find leaks is to use seafoam through your brakebooster line..same idea with this while pouring the seafoam in..have someone or you revving the engine while putting it in.. a lot of white smoke should be coming out of your exhaust.. if you see white smoke in your engine bay..that means you have a vacuum leak.