Symptoms of bad primary injectors?
 

I'm curious what symptoms "bad" stock primary injectors exhibit?

I suspect mine are not working properly. The car sat for a few years with gas in it.

Thanks.

I think that depends on the definition of "bad" because if they are just dirty from fuel sitting in there. B.G. 44K has done wonders for me including with my current FD for cleaning Injectors and such. I wouldn't consider that a bad Injector though, just dirty. This would affect idle and light throttle the most from my experience. Rough idle and jerky on off light throttle for really dirty injectors.

Usually injectors fail open which dumps fuel and fouls plugs or they fail and don't fire at all. this would be pretty obvious because the car would have a really hard time idling or driving since the secondaries Don't operate all the time.

you troubleshoot symptoms... don't look for symptoms and them try to apply them to your situation. always start with what its doing or not doing and work from there. what is your car currently doing or not doing that you would suspect the injectors?

symptoms:
- runs rough when cold
- "stuttering" between 2-3k rpm
- backfiring occasionally when letting off to shift
- occasional fuel cut type sensation when accelerating at part throttle but low rpm (2-4k) and low boost. This one scares me the most.

Just did new plugs and wires and re-installed the HKS twin power which helped the stuttering but it's still there.

Car sat for 3 years. Drained fuel, new filter. Replaced secondary injectors as they exhibited symptoms of adding too much fuel and overly richening the fuel mixture on the smog dyno causing me to fail smog. Now wondering if the primaries need to be replaced as well. They probably do...

backfiring could of course be TPS as well but that hasn't changed position in years so I doubt it. Will be checking TPS voltage when I reinstall the PFC soon.

an injector either works or it doesnt. the spray pattern can be not so great if its dirty or for any array of other reasons. have you tried verifying your fuel pressure is correct? assuming the pump was not replaced and also assuming you still have the o ring on the feed pipe, its possible your fuel pressure could be low. all that you have described also fits the symptoms of low fuel pressure. do you have a wideband?

Air pump working?

Dale

Thanks FDAuto. I have not checked fuel pump. Don't have a wideband or fuel pressure gauge on there atm. Will check. Had not considered that. It's a Bosch 044 that's been there for 10 or so years.

Dale, Yes, looks and sounds like it. Why? Seems like an odd thing to check given the symptoms. What am I missing? :)

This is what we found when we recently pulled my injectors. The seats that sit between the injectors and the dispersal plates had disintegrated.

(Not to mention some o-rings, pintels and vacuum lines. The damaged vacuum line was not delivering vacuum to the FPR which means it was closed. This may have helped overcome my dirty injectors to some extent.)

https://cimg5.ibsrv.net/gimg/www.rx7...a67961714f.jpg

"13-257"

https://cimg0.ibsrv.net/gimg/www.rx7...3eaaa1a437.jpg

I just went through restoring my rats nest and fuel system and made a video detailing the specifics and includes my parts list with part numbers. You can see it Here

I would definitely go through and restore your fuel system if it has not been done yet, if nothing else for general maintenance and peace of mind. That's what I did. My car was running fine, no issues but the parts are 30 years old, and I didn't want to take any chances with my motor before I even throw an intake on it given how the cost of engine parts have gone up in the past few years. I ordered 90% of my parts from Japan and saved a few hundred dollars since the yen to dollar value is down. I also did this because I figured the chance of getting a part that has been sitting on a shelf for 15 years was less since FD's are much more common in Japan and they are still regularly maintained and parts are still being produced for that market. Surprisingly, my old stuff was in decent shape, rubber on the injector insulators still bendable at least. Diffusers weren't falling apart. I only have about 45K miles on it which may be the main reason since I think heat is the main killer of these things.

If the car sat for that long with ethanol laced fuel, there is a good chance some "gunk" has built up somewhere (injectors being the choke points after the filter.) I would have the primaries professionally cleaned and replace all related parts while you are in there. At bare minimum while you are pondering this or saving up the money, try B.G. 44k, it smoothed out my idle after I first got my car because prior to me buying it, it sat for like 7 years before the car fax showed another registration and more mileage put on it. I threw in a can of 44K with about an 1/8th of a tank of fuel and took it for a spin to make sure the concentration was high, then let it sit for a few days to let it sit in the injectors to break down the gel, then filled up the tank and threw another can of 44K in. Idle smoothed out after that. I run a can in all my vehicles every 15K miles or so for maintenance, more often with the FD (every 3 months) because I don't put that many miles on it and I don't want gel building up. I've worked on so many small engines and boats over the years to see the effects of ethanol in our fuel and can rebuild many small engine carburetors with my eyes closed because of this. I only run ethanol free in my Generator and small engines now, I just wish we had a high octane option of ethanol free for our cars that require it, the highest I haver around me is 89.

The fuel pressure regulator uses both vacuum and boost to maintain fuel pressure. If the vacuum line had a leak that prevented it from getting vacuum, that same leak would likely prevent it from seeing boost. No boost to the FPR means fuel pressure would be too low, which means that a stock ECU would probably be running the engine leaner than intended. An aftermarket ECU might have been adjusted to compensate, running the injectors at higher duty cycle until the wideband O2 reading looks good. Either way, it's probably a good idea to have a wideband O2 gauge installed on a turbo rotary, and if your aftermarket ECU can use it for engine protection that's even better.

My understanding from the FSM and from the injector shop that recently refurbished my injectors:

The FD FPR may have a somewhat different function. In normal set up it is controlled by a solenoid that keeps the vacuum on (until it is told by the ecu not to). The vacuum helps modulate the diaphragm in the FPR. If the vacuum is absent the spring in the diaphragm pushes it closed, thereby backing up pressure into the injectors.

If the solenoid malfunctions or the vacuum line to the FPR is split, the FPR closes.

My car had the set up simplified, eliminating the solenoid and running vacuum directly to the FPR. Meaning the FPR was intended to be always open. Of course if the vacuum line is damaged, like mine was, the FPR closed.

This was likely one of the causes of flooding issue I had.

The vacuum line was running from the nipple that sticks out at a sharp angle from the lower, inner LIM.

Does boost and vacuum switch back and forth through that single line? I don't know. If yes, I'd like to understand that better.

My understanding is that my power FC relies on neither the O2 sensor nor my aftermarket AFR gauge.

Perhaps it relies on TPS and MAP sensor readings to modulate injector duty?

In any event, while I had the vacuum leak, the Power FC would hunt for about twenty cycles until settling on a steady idle.

It would repeatedly do this.

I am assuming that, each time, it was self-adjusting to the vacuum leak.

Subsequent to having my injectors redone, and my afr sensor/gauge replaced, so I could actually see the afr,, the car seems to be running quite rich.

I am assuming the Power FC was tuned to the somewhat clogged up injectors.

yes, but it only turns on when you start the car warm. so instead of vacuum, the FPR sees ambient pressure, so it raises fuel pressure for a bit
(the FC manual has a chart of coolant and air temp vs the amount of time the FPR solenoid is on, longest is like 90 seconds)

tee a boost gauge into the FPR hose, it'll act exactly the same as it did before, except if the solenoid is on, then it will read 0

PFC just follows the map you have, but you can map the car around broken/malfunctioning stuff.

So, how does one check fuel pressure on a stock FD fuel system??
I've only ever done that on my Spec Miata which has an aftermarket FPR and a port for a gauge...

get the inline pressure tester from the parts store. you T it into the line and it has a port for a gauge. its a whole kit

I have seen people do it where the fuel lines come through the firewall. There seems to be just enough space.

Make sure you put the clamps back on well.

One of my parts cars was a write-off, exactly for that reason.

"yes, but it only turns on when you start the car warm. so instead of vacuum, the FPR sees ambient pressure, so it raises fuel pressure for a bit
(the FC manual has a chart of coolant and air temp vs the amount of time the FPR solenoid is on, longest is like 90 seconds)"

Maybe we are saying the same thing. It took many reads of the FSM to puzzle this out, because it seems counterintuitive.

I believe the usual running condition is vacuum on, and for hot starts the solenoid cuts of the vacuum and the valve closes, blocking the return feed to the fuel tank.

As it is a closed system full of fuel, I am not sure how the outside ambient air pressure factors in.

For that matter, I don't really understand how my boost gauge switches from vacuum to boost while being fed by a single hose.

Unless, of course, my AEM Boost/vacuum gauge is being instructed by the ecu readings of the MAP sensor?

My assumption that the gauge it is reading conditions in the UIM directly via a non-check valved vacuum line being somewhat nieve?

Assuming you were serious, pressure/vacuum gauge:
The basic operating principle of a manual Bourdon tube gauge is a hollow, C-shaped tube which is opened to pressure at one end and welded shut at the other. When the tube is pressurized it tries to straighten-out, moving attached linkage and a pointer to indicate the precise pressure. When it undergoes vacuum, it curls up more, and the pointer indicates vacuum.

An electronic one just uses some form of a load cell.

this is a Greddy 80mm gauge, the pipe moves the levers and then the needle
https://cimg2.ibsrv.net/gimg/www.rx7...4d4c860549.jpg

I had no idea, thanks!

So if it is fed from a direct tube from the UIM, the gauge has to overcome vacuum to start reading boost? So for a reading of 7 psi boost, the turbo actually have to be generating 22 psi? Assuming the vacuum level is about 15?

In other words, what is going on when the boost gauge sits at zero?

If, like on my car, both turbos are spinning always, is the exhaust constantly being wastegated, when we are in vacuum?

No, the UIM has either pressure or vacuum, relative to ambient. With the engine off, there will be neither (gauge) pressure nor vacuum and the gauge (if it's functioning correctly) will read zero. At low loads the UIM will will be under vacuum, i.e., negative pressure compared to ambient. Under boost, the UIM has a positive pressure relative to ambient, i.e., gauge pressure. So 1 input is all the pressure/vacuum gauge needs.

The vacuum is created like a vacuum-pump sucking against a small opening, where the engine is the vacuum pump, and the small opening is the almost-closed throttle.

Note that ambient air pressure is ~14.7 psi relative to full vacuum. A full vacuum is zero psi absolute. In our case at the UIM, both gauge pressure and vacuum are measured relative to ambient. Absolute pressure is relative to a full vacuum, but that's not what we're measuring with a normal vacuum/pressure gauge.

To further clarify, a positive vacuum is actually a positive absolute pressure that's less than ambient. That's by definition a negative gauge pressure.

Thanks I sort of get it now. Let's bookmark this for when I ask again next year.

:icon_tup:

So extending this back to the FPR discussion above. In my case I have a vacuum line running direct to the FPR without a check valve. So the FPR is seeing vaccum/ambient and/or boost depending on manifold conditions. But once we cross ambient to boost, the FPR is already closed, either by spring pressure or boost pressure?

This could be leading to the overrich conditions showing up on my new AFR gauge?

I have to beg ignorance on exactly how the FPR functions - never studied it or needed to.

Here's a good animation and description of how the insides of a fuel pressure regulator work, it's a spring-loaded bypass, the spring sets the desired bypass pressure. The vacuum port pressurizes the spring, which is explained pretty well near the end of the video.

Here's a video showing how to rebuild an aftermarket regulator, the internals of aftermarket units should be similar to how the stock ones are built.

When I was searching for a good demonstration of how fuel pressure regulators work, I found a nice video from a well-known rx7club member. I'm not just including this because I'm a fan, but his demonstration showing why the fuel pressure needs to increase when the manifold is seeing boost pressure is very good.

The fuel pressure regulator is almost never completely closed, the bypass is spring-loaded and will open just the right amount so that the fuel pressure is about 40psi higher than the pressure at its reference port. If you're feeling adventurous and have a friend with a fire extinguisher on duty, try dumping the fuel return line into a 5-gallon fuel jug instead of letting it go back into the gas tank so you can see how much fuel is bypassed with the engine off or with the engine idling. On second thought, it's probably safer to just take my word for it.

It's probably important to note that many US-made 'boost/vacuum' gauges will use a different scale like inHg inH2O to display vacuum, because that was a common measurement unit back in the days with hot-rodders in the US were doing carburetor things. I'm going to use use psig units to describe both boost and vacuum because it makes the math much easier. People from outside the US might describe everything in Bar or kPa units, which are both good measurement units as long as you use the same scale for both vacuum and boost. When the reference port is correctly connected to manifold pressure, you can expect to see fuel pressure always be about 40psi higher than manifold pressure. When the engine is idling and pulling vacuum, the manifold pressure will be about -10psig (10psi less than atmospheric), and so the fuel pressure should be about 30psi (which is 40 higher than -10). At full throttle and zero boost, the manifold pressure will be about 0psig (same as atmospheric) and the fuel pressure will be about 40psi. When at full throttle and the turbo is making boost on a stock twins setup, the manifold pressure will be about 10psi (10psi higher than atmospheric) so the fuel pressure should be about 50psi (which is 40psi higher than +10). If you're interested, I can post screenshots from my aftermarket ECU that include manifold pressure and fuel pressure. The fuel pressure changes smoothly when manifold pressure changes, it's nothing like an on/off switch.

If the reference port is disconnected , it will never influence the bypass spring and the fuel pressure will not change as it should. When the engine is pulling vacuum -10psi, the fuel pressure would be 40psi (which is about 50psi higher than -10psi), and the engine will get more fuel than the ECU was programmed to deliver. At full throttle and zero boost, you get lucky and the engine happens to get the same amount of fuel the ECU was programmed to deliver. At full throttle and 10psi boost, the fuel pressure would still be 40psi (which is only 30psi higher than +10psi) and the engine will get less fuel than the ECU was programmed to deliver.

If your PowerFC was tuned when the FPR reference port was not connected to manifold pressure, the tuner would probably have increased the injector duty cycle to compensate. I forget the exact math, but my guess is around 10 or 15% more duty cycle would be needed. If you fix the FPR reference after the ECU was already tuned to compensate, the wideband O2 gauge would probably show the engine running a little too rich in boost.

PRC Solenoid
 

So, Mazda inserted a solenoid into the vacuum line running from the manifold to the FPR, thereby adding an element of AI and potential for trouble. It seems the ECU considers five inputs to decide to tell the solenoid to cause the vacuum to be choked off..


They make it hard to find in the FSM by calling the Solenoid valve the PRC Valve. But it can be found on page F105 of the FSM. Or here:

https://cimg8.ibsrv.net/gimg/www.rx7...d291b61f75.jpg
What could possibly go wrong.

Page F-31 of the 1993 "Service Highlights" gives us a little more explanation about what is going on.\

So what happens if your 30 year old PRC Solenoid decides to pack it in?

https://cimg3.ibsrv.net/gimg/www.rx7...c4358d7445.jpg

nothing, solenoid is only turned on during a hot start. the rest of the time it is off. so if it fails, maybe you have a rough idle after the car starts hot.

its labeled PRC in the sensor check screen of the Power FC commander, you can watch it if you like

I deleted the PRC solenoid from my car about 15 years ago, and have an aftermarket fuel pressure regulator set to about 50psi base fuel pressure. 50psi was per the recommendation of an rx7club member who had worked closely with Paul Yaw, he said ID1000 injectors would have a better spray pattern at 50psi than 40psi base fuel pressure. Better spray pattern helps mix the fuel with the air so the air/fuel mixture burns well after the spark ignites it. I haven't noticed problems with hot starts, aside from needing to adjust the ECU temperature-based fuel trims from the default settings in the base map. The odds of an ECU base map being perfect for a daily driver are slim, at least for the brand I have. I've also added a fuel pressure sensor, so the ECU can adjust the injector duty cycle to compensate if the fuel pressure goes higher or lower than expected. Many modern cars have fuel rail pressure sensors, especially turbocharged ones.

Here is a table of what each solenoid is doing when. PRC is near the bottom. I suppose the chances of it getting stuck "on" are slight. But broken nipples are more of a possibility.

https://cimg4.ibsrv.net/gimg/www.rx7...340724ce83.jpg