Non Z, but Nissan.. V6 pathy runing poor..

[datsunlover]

Not a Z, but beeing a Nissan, I'm hoping someone here might be able to help me with this.. My bush/winter beater isn't running well, and I have to get it to pas an emisions test soon..

Anyone know if the Pathfinder (89' 3.0L V6) has a 'limp' or 'failsafe' mode in the ecu? Heres what it's doing; Starts up ok, but idles high (about 1200 rpm) and has a miss/pop every few seconds.. Also, it wont rev past 3000 rpm.. hence the limp mode question. My 75 Z had this, and wouldnt go past about 2500, so I suspect it is a similar issue.

Now, I should state that it needs an oil change, probly plugs/wires, and an O2 sensor. (there isn't one in there at all right now.. stupid previous owners..) Thing is.. the check engine light isn't on, and aside from dying at 3K rpm, it drives pretty good.. Im thinking maybe it's just bacause of the lack of an O2 sensor the ecu is confused? Really.. I think it should make the check engine light come on though.. any thoughts?

[zcarnut]

I do not own a Pathfinder, but I have a V6 Hardbody truck of the same vintage with the VG30E engine. Yes, the VG30E motor has a “limp-home” or a fail safe mode which allows engine operation in the manner you described.

 

However, several items can result in this mode. A defective mass air flow meter, a defective crankshaft position sensor, a defective throttle position sensor (TPS), a defective ECSS module, etc.

 

The factory service manual has a complete diagnoses section where you can read the blinking LED’s on the ECM and determine what the trouble codes are. This will identify the problem sensor. There are way too many codes to list in this short reply. If you want to fix this yourself then I would strongly suggest to get the factory manual.

 

Yes, the CHECK ENGINE lamp should be on. I bet the previous owner removed the bulb. The CHECK ENGINE lamp should always light up when the engine is off but the key is in the ignition position (bulb check).

 

If you want additional assistance check out the below forum:

 

http://www.4x4parts.com/ubbthreads/ubbthreads.php

 

 

P.S. Your 1975 Z car did not have a limp-home mode. Not be able to rev past 2750 rpm was almost always the result of water entering the TPS.

[datsunlover]

Thanks for the info and the link, I'll check that out. I didnt realize the ecu would have that light on it (my 85 200sx did.. figured they got rid of that feature) I'll have to get a manual for it. As for my 75, uh.. yes, it had a 'limp home' mode.. A Nissan parts guy (who was a Datsun mechanic back when these cars were NEW 25 years ago) explained it to me. He also said to check the TVS (switch remember, not a sensor) and I did change it out, but to no avail. Doesnt matter now anyway as I switched to SU's. Thanks again man.

[zcarnut]

I didnt realize the ecu would have that light on it (my 85 200sx did.. figured they got rid of that feature)...

 

Yet another reason to own a Nissan. Lets us “backyard mechanics” do our own diagnosis without having to use a PC or go to a dealer for service.

 

As for my 75, uh.. yes, it had a 'limp home' mode.. A Nissan parts guy (who was a Datsun mechanic back when these cars were NEW 25 years ago) explained it to me. He also said to check the TVS (switch remember, not a sensor) and I did change it out, but to no avail...

 

TVS = Throttle Valve Switch. Senses Idle Position (throttle closed) and Full Throttle (throttle open or almost open). Sometimes people interchange the terms TVS and TPS.

 

What was happening on your ‘75 is this:

 

Under deceleration conditions the fuel injection electronic control unit (ECU) shuts off the injections. This is done mainly for economy and it also helps to reduce emissions. The ECU uses the Idle Position switch to accomplish this and also the rpm of the engine. Under normal working conditions the injectors are shut off when the engine rpm is above 2800 and the throttle is closed. Below 2800 rpm the injectors begin working again.

 

So, if the ECU reads the TVS as being always in the Idle Position then the deceleration algorithm prevents operation over 2800 rpm. The presence of water in the TVS completes the circuit even if the TVS is mechanically OK. Even water in the TVS connector (entering through cracks in the boot rubber) will produce this condition.

 

I do not wish to belabor the issue, but what I are trying to point out is that your 1975 Z did not have a “limp-home” mode. Limp-home or fail-safe mode algorithms can be very complicated as the ECU has to “substitute” the missing information from a particular sensor. This is far easier to do with digital ECU’s (using lookup tables) than with the analog ECU’s that were used in the 1970's and early 1980's. The 1975 Nissan ECU was developed under licence from Bosch is almost a direct copy of a Bosch ECU and it is well known that “limp-home” modes did not exist in the early Bosch systems.

 

Limiting the engine rpm in “limp-home” mode is done only to alert the driver that there is a problem with the engine that requires attention. Otherwise the driver would continue to ignore the CHECK ENGINE lamp. The fail-safe modes are good enough that the engine would run satisfactory. Most people will not have a vehicle serviced until it is almost completely inoperative.

[datsunlover]

Hmm.. ok then.. what ever it was in my Z, doesnt matter anyway! Got carbs now; easy to tune! Im thinking though.. my pathy problem may be a tps/sensor then.. I have a parts truck (identical year and model) so maybe I'll try swaping some parts around. The check engine light comes on with the key, but goes out after start up so it's working.. But will the light come on if the ecu is in 'limp mode'? IE; shouldn't the light be on FIRST, before the ecu switches to limp? I've got to get the manual so I can get the codes out.. and put in a 'known good' (as in NEW) O2 sensor. Ive got one from the parts truck in there right now.. I'll let you know how it goes. Thanks again for the help.