Check engine light 20 mins after entering the Interstate

[goldeneye]

Hello,

I have an Infiniti Q45 1994. I bought it in Feb 2005 used with 100k miles on it. For a year, it ran fine. First thing I did after buying was flush xmissionm checking brakes, etc. 1 year later the radiator died and was replaced. I am a grad student so the car is mainly used from Home-School-Home trips during the week, and long roadtrips during holiday. I took the car on my ever longest roadtrip, Tucson, Las Vegas, Lake Tahoe, San Francisco, Los Angeles, San Diego and back Tucson and the trip went fine without a glitch with 4 people on board and luggage to last for two weeks during winter. Brakes, master cylinder was replaced before failure without incident.

A year ago, driving to a conference in San Diego the car lit the Check Engine light on the trip after almost 1 hr on going 75+ mph on the interstate. The light would just turn off after turning the car off and back on in city driving.

Back home, nothing wrong could be found on the car and since in the city the light was not on, I kind of forgot until emission time came, where the car passed but I noticed (I am kind of OCD) that the numbers were much higher than last years. After that, a couple of months later one injector gave up (after smelling gas in the cabin for a few days), and I, being cheap decided to gamble it and just change one. Two months later another two died (Murphy's law). Luckily enough the mechanic gave me warranty on the previous work and I decided now to change them all 8($1k+ including labor).

After that the car driving improved immensely as well as fuel economy (if you can call it economy in this type of car hehe). But last weekend I took a short (150mi) roadtrip and again the "Check engine light" lit again after about 20 -30 min of driving at 75+ mph. I have a bearly noticeable shaking at around 75-80mph which goes away driving below or over that range of speed. Also, once lit, the light will turn off for periods of 10 - 15 min and back on for another 10 to 15 min. Back to Tucson and city driving is still dream like. So sorry, I know is kind of stupid but I didn't actually made a point of trying to remember the RPMs, I would guess that the car was always between 2k and 3k but that's kind of broad.

Any ideas what could I have wrong?

Thanks

Juan PD: by the way, its probably obvious I am not a mechanic, I am an Electrical Eng.

[AlabamaDan]

I have a similar problem with an intermedant check engine light. The first time it happen I was barrelling down the Interstate at about 80 and called my trusted mechanic (not to be named but a good inifiniti guy) who said it was an emissions thing not to worry about it. That was about 3.5 years ago.

I really need to see what code it throws. What say everyone else?

[CrimsonQ]

Can you check the codes?

[szh]

Can you check the codes?

BTW, if you had a 1996 or later, you can rent (for free) a scan tool at Autozone to get the codes.

Note that this a bit more complicated on a 1994. This is pre-OBD 2, so you have to follow the procedures (that are available here in our technical articles section) with the ECU. These are not beyond the skill set of an EE!

Z

[maxnix]

If you are an EE, check the O2 sensors duty cycle on the scope.

Q45tech has a great post pertaining to this.

[goldeneye]

The codes, I will have the mechanic that changed the injectors download them. Also, excuse my ignorance , but for how long do the codes remain in memory? The check engine light is not on in city, and the 'incident' happened after some time driving in the interstate.

With respect of the O2 sensors duty cycle: can someone post a link to the thread that from Q45tech that talks about how to, maybe I can do that this saturday. What is the freq./duty cycle/etc I am looking for, probably in the range of 10s of miliseconds?

In the city the car doesnt give me any reason to think something is wrong. My whole injector trouble started with the chk engine light but also a strong smell of gas, which i dont have now.

Thanks

Juan

[Rex]

Codes should be stored for the next 50 "starts", IIRC.

Here's how you can pull the codes yourself

http://www.nicoclub.com/articles.php?id=60332

get the codes and check back.