Hello everyone, I've got a troubleshooting question about my fuel system. Kind of a long post, please bear with me.

I've got a '93 NA automatic, all stock, in fair condition with what appears to be a JDM engine of unknown origin, 151,000 miles on the odometer. I've been slowly massaging it back into shape over the last few weeks.

Engine performance is really inconsistent. Most of the time the performance is acceptable. Sometimes it'll take off like a rocket from a stop or a downshift, and I wish it always ran that way, and other times it falls flat on its face!

I've pretty much covered the obvious stuff like putting the NGK's in it, cleaning the MAS, changing air and fuel filters, cleaning all electrical connections I could get to, etc. It's improved quite a bit from the day I bought it.

The idle is rough regardless of temp. It performs pretty good when cold and slightly warm, but when it gets hot, the performance takes a HUGE nose dive. The kind that makes it unsafe to drive in traffic. It'll rev up to redline, but will take forever to get there. As if I were towing a heavy load or something. And it happens rather suddenly, as in there isn't a steady decline in performance as it heats up. It just gets to a certain point after about a half hour or so, and then the performance is ****. Like I'm driving a worn out Civic with a plugged cat or something. I notice a little chugging under acceleration when its like this, too, but its not very noticeable.

Fuel pressure at idle is about 38psi, and off idle it jumps up to about 44, but then settles to about 40 and stays there. I didn't think this was normal, so I disconnected the FPR vacuum, and it jumps to 44psi steady at idle. Off idle it jumps to 50psi and stays there. I swapped out both the FPR and Dampener for known good ones, and got the exact same results, so I really don't understand why the pressure jumps to 44psi, and then falls down to 40 when I open the throttle....or if that's even related to the poor performance when hot.

Radiator is full, coolant temp sensor is clean and in good condition as far as I know, and the temp gauge never strays from the middle when warm.

Resistance on all injectors are at or around 10 ohms, (+- .2 ohms) when engine is cooled down.

TPS voltage checks out and is smooth from closed to WOT, when cool, engine off.

My next guess is to buy a longer hose for my pressure guage, and see what it does after I've been driving for a while to see what the fuel pressure is doing under acceleration when hot. Not sure if I'm looking in the right direction though.

Any insight or suggestions would be much appreciated.

Thanks

 

Anybody know if this performance loss could be due to a bad Detonation sensor?

If the sensor fails, does the ECU SEVERELY retard the ignition timing and keep it there as some king of safety measure?

I get an intermittent CEL for Detonation sensor circuit ever since I cleaned the sensor connection.

 

At this point, I think it is the sensor and/or the harness. I'd put my money on the harness itself, since I never noticed a problem until the day after I cleaned it.

Today, I actually felt the point where the power loss kicked in after pulling away from a left turn. I slowed down to 2nd gear and opened to 1/2 throttle. Sure enough, engine was limp. But never threw a CEL!!! I pulled over and checked the codes, and got code "34". (That's 2nd time it threw a code 34 without a check engine light). I cleared the codes, got out, fiddled with the connector harness, and started the engine. Took off like there never was a problem, and made it home without noticing another issue.

Still doesn't explain why my fuel pressure jumps to 44, then drops to 40 psi when I rev up to 2000 RPM....

 

I found some confirmation of this on TT.net by searching for "code 34"

He's talking about a twin turbo, but I assume this portion of the ECU's knock detection programing is similar for NA's.

quote from article by AshsZ (FABio) on TT.net:

"Now, there are two modes of code 34. Short term and long term. I'll start with the long term mode because it is much shorter to explain.

If the ECU detects that there is a problem in the detonation circuit itself, it will trigger a long term code 34. This will cause the ECU to revert to the low octane fuel and timing maps until the circuit is fixed (replace the sensor or the harness, or both) AND the ECU MUST be reset to clear the code....."

I didn't replace anything, but I did reset the ECU.

--and now, Ash's explanation of the short term code 34 (edited for brevity):

"If knock is detected, .......... then it will reduce timing by one degree and then fire the [spark] plug. The next time around, if it finds that there is again detonation, ............. it will reduce the timing by two points from the stated point in the map ............. If it continues on like this in detecting knock over and over again, it will reduce the timing by as much as 4 degrees. At this point,..........., it will kick the system into safety boost and it will now be using the low octane maps. This is now in short term code 34, safety mode. This condition can be reset by simply turning off the car and waiting 10 seconds, then restarting."

link to full article:

http://www.twinturbo.net/net/viewmsg...ral&dtSearch=0


He explains that when all is good, and the ECU is running the high octane map, it can run up to 24 degrees of timing....but once it's in safety boost mode it will only run a maximum of 16 degrees, and if knock is still present, will continue to retard timing EVEN FURTHER. Also, it dumps more FUEL when running in safety boost mode. (I burned thru 1/8 tank while driving around my neighborhood for 20min).....This could explain why my engine looses so much power so suddenly when its hot.

So I figure I probably have a few things happening, including.

#1) Either the Detonation sensor and/or circuit are truely failing, and the ECU is reverting to this safety mode map (which I would assume would throw an actual CEL on the dash).

#2) Detonation sensor/circuit is fine, and I'm incurring so much knock when the engine is hot, that it's going into "short term code 34" (and not throwing a CEL on the dash). If this is the case, then why so much knock??

I've also noticed that my rough idle smooths out when its in this safety mode. So perhaps the extra fuel being delivered in safety mode is compensating for unmetered air entering the engine at idle from vacuum leaks??

If #2 is true, then perhaps I'm getting so much knock when hot, because of the possible low fuel pressure issue I mentioned in my first post (pressure drops from 44 to 40psi with throttle open), causing a lean condition every time I step on the gas.

PS. any Z guru's feel free to jump in here if I'm off in my logic. lol. I can get lost pretty easily.