Puzzle: vacuum leak, injector crud, or O2 sensor fail?
 

After a month of normal and flawless operation, I am back to the very same symptoms that caused me to replace the fuel pressure regulator. No CEL, no pending codes. But the ECU thinks the AFR is off and is therefore cutting fuel trim, sometimes at much as 35% and often 10-25%. when I hit the throttle, trim will be slightly increased, which seems normal for this Honda. Then trim steadily decreases fuel as I hold steady throttle, at either high or low load. It will even do this at idle. Though it has bucked a little once or twice, it has never stalled (yet).

My power is reduced. Fuel economy is suffering badly.

Last month I tested fuel pump pressure (normal) and fuel pressure regulator (normal). Replaced fuel filter (it was past time). Compression test too was good.

Yesterday I removed each spark plug, all black around base and gray around insulator.

No smell of gasoline in any cylinder.

Vaccum leak? MAP sensor? fuel injectors? O2 sensor?

I replaced the O2 with a good quality unit nearly 3 years ago. It has maybe 40,000 miles on it.

At this point I decided to drive it until it fails or I get a CEL. I have AAA+ and therefore free towing up to 100 miles.

I figure I'll patiently test items. Learn. Fix. I need my DD.

Any advice appreciated.

james

Cold start this morning, it idled raggedly in open loop, and then steadied at idle in a few seconds once the UG indicated closed loop operation... at the same time the trim started getting lean. It stabilized in the negative teens (-14 to -18%).

Once warmed up it stalled once while coasting with the engine on in neutral.

There was one episode where the primary O2 reading on my UG showed voltage 0.995 for something like 5-10 seconds straight with no flutuation.

Warm, the engine generally idles comfortably in the 600 rpm range, with a fuel trim -24% or so. Sometimes it idles almost at normal 0.00 fuel trim.

more reading, more thinking outloud
 

Maybe I can rule out the vacuum leak idea, because if the ECU is reducing fuel the must either be more fuel getting into system than ECU expects or less air. A vacuum leak would mean more air getting into the system, which the ECU would adjust to by adding fuel, not subtracting it.

Maybe I can also eliminate the O2, since the problem occured during open loop in cold start this morning. In closed loop the idle stabilized as the system detected a rich condition and took away fuel. Maybe eliminate the MAP sensor too, for the same open/closed loop reason.

This leaves fuel injection or a fuel leak from somewhere.

I have had a similar problem in my Escort. What I was told by one of the respected members at TeamZX2 was it is a vacuum leak. A bad MAF/MAP would cause + fuel trims. A vacuum leak causes - fuel trims. I think it is related to the EVAP on mine, or PCV lines. I know you have a Honda, but these are worth a shot considering our issues are similar.

Thanks. But a vacuum leak would mean more air getting in, which would mean the ECU would add fuel, no? That would mean positive fuel trim, and I am seeing negative fuel trim.

Try some techron first. Uneven injector delivery volume or poor spray pattern could be an issue. It could be MAP related, trim is probably through the 02 feedback. Reading almost 1 volt is very rich if my memory still serves me.

regards
mech

You are correct, good sir! I got them switched in my brain before I typed them :o

- FTs is bad MAF/MAP
+ FTs is vacuum leak.

Sorry for the confusion!

Look at it like this, how many ways is it possible to get too much fuel for the proper amount of air.

I've seen the injectors in a Z car that were continuous when they were supposed to be momentary, ecu fried by a reverse polarity jump start, another time fried by leaking windshield. The ecu regulates fuel delivery through the ground. Obviously a continuous ground would be a big problem.

Listen to the injectors with a stethoscope compare each ones sound to the other. If one is silent, clean the connection and try it again. I've seen engines not miss with a dead injector, the fuel trim was increased by the remaining injectors until it provided enough fuel for the car to actually run better than you would think.

Injector issue
ruptured diaphragm on the fuel pressure regulator
Open circuit on the ambient air temp sensor (would massively flood out a z car in two seconds).

Now here is the real problem. Even if you replaced the fuel pressure regulator yesterday, it could still be defective, even if you replaced it three times in the last week.

Had a customer replace 4 starter motors in his Z before he came back to me and apologized for calling me a thief, after I tried to explain the difference between the $39.95 direct drive cheap rebuilt for Advance, versus the $129 Nissan factory rebuilt gear reduction starter motor.

This is one of the reasons why I retired

regards
mech

Amazingly, I appear to have solved the problem from a "surprise" direction. I was wishing for a mechanical vacuum gauge because it seems odd to always rely on the mediation of the UltraGauge or my OBD2 diagnotic reader thing. Then I remembered someone saying cleaning old grounds can help the car run. Then I remembered cleaning the grounds has been on my list of long-procrastinated maintenance for a year or two. Then I took 30 mins to clean engine ground, transmission ground, battery ground, and one other grounding cable. And.... PROBLEMS ARE GONE! Just pulled nearly 70mpg in traffic on a 30 mile test run. Mostly normal fuel trim for a car doing a lo of P&G (slightly on the + side by 2 or 3%). Wow. They had not been cleaned since 1998. Hahaha!

BTW, I also got fuel system cleaner, Mech. What the hell, it can't hurt. I bought a big bottle to use in each tank for a few months. Thanks!

Use marvel mystery oil as a fuel additive it works great in Hondas