I30 bogs down at 2000 RPM

[jvanwyhe]

I have a 96 Infiniti I-30, V6, 5 Speed Manual.

Once the car is warmed up, it will drive fine, under 2000 RPM. Once the engine revs to 2000 RPM, it cuts out (but does not stall), and will barely run until the RPMs drop to below 2000.

Another syptom that has occurred with this car is that occasionally the speedo will instantly drop to zero, for no apparent reason. The speed sensor was replaced at a dealer, but that did not fix it, and they had no idea what the problem was. I left it that way because it was only a minor annoyance, and happened rarely (and for a short period of time)

I pulled the engine codes, and came up with 0104 (speed sensor) and 0304 (Knock sensor). I tried the knock sensor resistor trick just to see if that was the problem, but it didn't work. I also pulled the plug and ran it without - same thing.

Now before I go and replace every sensor on this thing, does anyone else have any ideas?

It seems to me that these problems may be related. Could I have a bad wiring harness, and if so, how can I check it?

Could the ECM be bad, throwing out unrelated codes? If so, how can I check it?

I'm not cheap, but this is my drive-back-and-forth to work car, and I don't want to dump big $$$ in an old car. I would just replace the whole thing instead.

[Nahkapohjola]

Originally Posted by jvanwyhe

....and I don't want to dump big $$$ in an old car. ...

When certain sensors fail, engine drops into safe mode (2200rpm ceiling). MAF, CamPos sensor... NOT knock sensor (only ign retard). Test the sensors. For wirings u need schema, try and find info for u car... Follow links http://www.cardomain.com/id/pohjola#links

...sensor testings info, connector cleanup p.9 & 14:
http://www.cardomain.com/ride/748507/1

.

[jvanwyhe]

I just wanted to post a follow up on this for anyone encounter similiar problems.

Nahkapohjola was indeed correct. My knock sensor was bad, but had absolutely nothing to do with the engine getting thrown into safe mode by the computer.

While working on the car, I noticed something strange. When I started the car, all the cooling fans came on, even though the car was ice cold. This, I knew, could not be correct. So I immediately traced through the cooling fan circuit (using the Haynes manual troubleshooting) to the Coolant Temperature Sensor. I thoroughly tested it, and it came back within tolerances shown for resistance.

But something didn't seem right, so I "forced" correct values by pulling off the connecting plug, and putting in resistors (similiar to the knock sensor trick detailed several different times on this board). First I cleared the computer. Then I used a 2 resistors (500K total, in series) to simulate a cold start input, and I used a 100K resistor to similuate a "hot" input. Surprisingly, the fans turn off the cold start (as they should), and the car was no longer safe mode either time. So I replaced the sensor, and $20 later, the car runs like new.

This exact same procedure could also be used to troubleshoot an Intake Air Temperature sensor as they also use a thyresistor setup and the resistances used are identical.

It is important to note that even though no codes were shown for this sensor on the computer, and the sensor (supposedly) checked out, it was still bad. Anyway, this is a cheap and quick way to check a couple of sensors if you have a similiar problem.

[Nahkapohjola]

Originally Posted by jvanwyhe

I just wanted to post a follow up on this for anyone encounter similiar problems....

Good to know, thanks.

The sensors inputs that cause 'safe mode' do vary. Different ECU's have different programs as I cannot produce 'safe mode' on my 3gen VGE while playing with temp sensors (fans will hit on).