low vacuum question

[ohvictor]

I've got a stock 93 Ka24de with headers and intake, but recently its been running like crap at idle and at cruising engine speeds but during acceleration its smooth . It kinda sounds like it has cams. i checked my vacuum and it was at 12. i opened up the oil filler cap and the rpm was somewhat smoother so i checked vacuum again and it went up to 19. Any one know what could cause vacuum to be so low at idle? And a ways of more testing would be much appreciative.

Things i've checked/replaced

-I've spent a good hour + replacing the Pcv thinking it was clogged but still no luck.- lifted the bottom of the EGR diaphragm, that caused the motor to bog and die out .. so im guessing its not stuck open?-fuel pressure is 41psi at idle .. but im guess its cause of the low vacuum at idle -I used a MAF off a 97 240sx .. the pig tail didnt fit but i just connected wires across the harness to the sensor and it acted the same-replaced the o2 sensor cause the wire melted from hanging low next to the exhaust-replaced injectors/rail/fpr from a 97 240sx-swapped ecu's with a known good one-removed and cleaned my IACV-AAC and before that i let the motor sucked seafoam into the vacuum line going to the Iacv-aac-checked exhaust back pressure, it wasn't steady so i took off the cat, it wasn't clogged but i ran the car with it off and it still sounded like a subaru -Compression check was good

so i'm kinda stumped as of what more to check and How to go along checking it..

[Rucca]

Have you ever messed with your timing, especially cam timing. You say it sounds like you have cams - if your cams are different you can change idle considerably. Also, check ignition timing.

[ohvictor]

Have you ever messed with your timing, especially cam timing. You say it sounds like you have cams - if your cams are different you can change idle considerably. Also, check ignition timing.

never touched it, but i've just checked timing its at 20 when i cause a vacuum leak that brings the vacuum back to 19 from 12. when vacuums 12 timing and my idle is erratic.

[ohvictor]

this was weird too, i had the car running and went to take off the fuel pump fuel to kill pressure so i can hook up my fuel pressure gauge again. and my car ran normal, it wasnt running rich and the idle was steady, for a good couple of minutes till i decided to turn off the car and turn it back on to see if it would start up but it didnt.

i checked for codes stored in the ecu, but it game me the OK code.

besides that i read to check for resistance in the battery ground wires so i'm gonna give that a check tonight.

any more ideas?

[Rucca]

Restricted exhaust? Dunno.

[ohvictor]

Restricted exhaust? Dunno.

that was my first guess but my i could see through the cat, it looks normal and not melted

[hugesack]

check intake manifold over real good for cracks. try spraying carb cleaner around the gasket area to make sure you dont have a leak

[ohvictor]

check intake manifold over real good for cracks. try spraying carb cleaner around the gasket area to make sure you dont have a leak

well i tried that and nothing happened, i doubt its a vacuum leak causing my problem because i open up a vacuum line (making a vacuum leak) at the brake booster and my vacuum increase from 12 to 19.

[hugesack]

also check your fuel pressure with hose disconnected. your fuel pressure should be at 43 psi with the hose disconnected.

[ohvictor]

i was searching nico to help see if anyone had a similar start and i found this link, im gonna try to see if i can borrow "consult" or hopefully the MAC tools truck has something similar on their truck today =] because it all started i would say after i put the dc headers on my car it didnt come with a o2 relocation kit cause i got it off Craigslist so i made my own wire and it melted i didn't notice until i started to get a stumble on decel and backfire as well. later on it got worse and worse now my gas mileage is crap . i was talking to my boss and he said if i'm running rich enough to cause a misfire that it would also effect my engine vacuum.

its worth a try but im still open for suggestions

zer...19228

I guess I should update this for anyone who searches for a problem similar to mine.

To sum it up short, it was a bad 02 sensor.. that went bad last year, but the ECU went into a learn mode and covered up any symptoms of a bad 02 sensor very well (no power loss, and it retarded it about 5 degrees so no real noticeable difference). It wasn't until I went to the DMV for what I thought would be a routine tailpipe pass, that i realized there was a problem.

So I replaced the 02 sensor since everything else was fine because the wiring did melt in two places and caused the sensor to ground out (initial problem to cause it to run rich, thus burning up the 02 sensor).

The problem didn't correct itself w/ the new Bosch 02 sensor and even after fixing the timing. My bf no longer was working at Nissan as this point and no access to the Nissan Consult diagnostic computer. The ECU kept retarding the timing even after he changed it back to stock. The car kept running worse..

so I bought the Nissan Datascan (consult copy) from or sponsor (blazt-something), figured it out, hooked the car up one day, and used the set timing mode in the software. It took 4-5 tries to make the ECU accept the new values. It finally did and it was running much much better, more power, etc..however at this point the new 02 sensor burned out in two weeks (bosch sucks btw) and it was still hesitating. So I bought another one, this time a Denso 02 Sensor (direct-fit and only $22ish at rockauto.com) and the hesitation went away. We checked it a few more times after that, and I decided it was time to try the inspection lane again.

It passed with absolutely flying colors! My car did a .36 and .47 for Hydrocarbons (compare with what it did in my first post!) .... eat that Honda!

So yeah, once the ECU goes into a learn mode you have actually force the new timing values into it w/ the 'set timing mode' via the diagnostic software.. there's no other way.