sharingan 19

iTrader: (16)

The Basics:
9.4 rotors, s5 t2 irons, s5 n/a housings, Goopy seals, Mazda everything else. Stock s5 turbo with ported wg running 5psi. R-tek 2.1 running conservative timing maps. 5k miles.

The Problem:
I went down to Tampa to visit family and friends, Avg 17.4 mpg. Decided to do a Christmas eve run w/ some buddies on some twisty roads, so I filled up and burned through about 1/3 tank boosting up and down the road. On the way back I notice my vac had dropped from 20"hg @ 800rpm to 18, although the car ran fine, I was a little concerned. Imagine my surprise when I fill up to head home and discover my mpg has climbed to 18.4! If course I thought this a fluke, so I dismissed it, only to find on the return trip avg 20.3! Something happened ripping up those back roads, and it wasn't a Christmas miracle. Unfortunately...

I was headed back to Tampa a few says later, getting on to the road in 2nd gear, and right as I did a car popped out behind be attempting to pass a truck. I floored it to get up to speed, shifted to 3rd and noticed a lack of power and a slight hesitation. I shifted to 5th and coasted to light where she stalled and refused to start. (I was monitoring boost the whole time and it never went past 5psi, afr never went past 12.1).

I cut power to the fuel pump and was able to get her to turn over but of course it died immediately. I reconnected the pump and she cracked but did not start, until I held the pedal to the floor, then reluctantly she fired up and idled; very poorly. I had to manually keep the idle up to avoid stalling.

To avoid the cost of a tow truck I decided to limp her home. Surprisingly she ran great on the interstate (70+ mph @ 3k) and when I got home she idled; 12-18"hg @ 600-900 rpm (previously it had been consistently 18 @ 800).

The next day I went to see if she would start, and she did so easily. The idle didn't hunt, just settled at 16"hg @ 750rpm.

Thoroughly confused, I ran a compression test, front: 90-90-90, rear 60-60-30. That would suggest a side seal. However it runs better than I would expect for a blown seal. I mean other than taking a few more rotations to crank up, she runs just as good as before (although I haven't been boosting)

The Question(s):
What could cause an engine to lose vac and pick up mpg?
Could that be related to loss of compression?
What would cause these symptoms?
What (if anything) can be done to fix them? - rebuild is not an option currently
Is it safe to boost?
Also, the engine builder was lazy and didn't clean the carbon off the rotors, is it possible that some came loose and compromised the seals?
Any questions, suggestions or constructive comments would be greatly appreciated.

FelixIsGod29X

iTrader: (50)

seafoam it and see what happens considering the PO didnt clean the carbon off the rotors.

fcrotory

iTrader: (20)

Thats a horrible idea. Carbon could break off and do even more damage to the motor.

SpeedOfLife

iTrader: (16)

^^^ he's right, that could lead to disaster. It may not, but it's certainly a risk.

It could be a combination of things. A vacuum leak could cause some of the issues, stuck seals could cause low compression yet still run relatively well, and highway mileage can vary a lot depending on the wind and such. I've seen 25mpg highway before on a long trip with my S4 TII, but it's often more like 20-21mpg highway.

60-60-30 is really low, I'd check again making sure it's warmed up first. I'm not real sure how or why it would idle well with ok compression up front then such terrible numbers in back, so I wonder if the test was somehow a fluke.

K-Tune

iTrader: (26)

Does the idle SOUND normal? Does the vacuum needle twitch at all during idle?

Just trying to rule out a chipped apex seal.


If the builder was too lazy to clean the carbon off the rotors I wonder what else they were too lazy to do...

RotaryRocket8

Wow, really well writin' post. Rare to see on here to be honest. You know enough to ask the right questions and are very detail oriented.

With that, I am watching the responses closely to see what "I" can learn from this. =D

Good luck friend.

sharingan 19

iTrader: (16)

Would it be any more of a risk than doing the water/sea foam treatment on tired 100k engines it cars that have been sitting fir years, as is commonly recommended?

This is not normal mileage variance. For 8 months the mileage has not varied by more than 1mpg. Low 18's city high 18's highway, even with hooning around in a parking lot. Which I always thought was strange, since stock is 17/25 you would expect if it could muster 18 city, the hwy mileage would likewise improve (i too saw 25mpg a few times on long trips w/ the stock engine). About a month ago when the weather started getting cooler mileage began dropping as well, down to 17mpg. So jumping 3 mpg in 3 days is not normal.

I did the rear test first actually, after driving over to a friend house. It was at operating temp when I arrived and I let it cool for 10-15 min. I used a piston tester w/ Schrader valve removed. I used the leading plug hole and battery voltage was 11.X Funny thing is, I did the auditory test and the pulses sounded pretty even. Maybe it was a fluke, although I ran the test twice with me looking and twice w/ my buddy looking.

sharingan 19

iTrader: (16)

Once it's warmed up it sounds fine, the needle doesn't twitch either. That was not the case right after it happened however. It fluctuated between 600-900 and 12-17"hg and often wanted to die.

What would a chipped apex seal do?

Yeah, exactly...how bout not remove the n/a exhaust baffles before building the engine, and grinding them out after the fact...

sharingan 19

iTrader: (16)

Thanks, having attempted to help more than a few people based on half-assed requests for assistance I subscribe to the mantra "do unto others as you would have others do unto you"

Hopefully we will both find the answers we're looking for.

bumpstart

im suss on the test.. should be done at trailing hole and both rotors compared under same conditions on a dry engine
its possible one rotor is wetter than other and gives a false result

my immediate thought is you have a lazy leading plug and possibly some carbon lock issues
and these can be easily sorted
.. use water to siphon in while engine is running to steam clean it
then change to new leading plugs , run and warm up engine.. shut it down then perform test
use shraeder button in bypass for bounce count
use shraeder not bypassed for peak number

trailing plug location.. throttle wide open .. fuel pump fuse pulled

j9fd3s

iTrader: (3)

agreed. we had a pretty dramatic example of this at the dealership once. in about 2007 an Rx8 came in, running on 1 rotor, we did a compression test, and it was in the mid 5's, which is failing by a lot. Mazda had us replace the coils/wires/plugs, and once the thing was restarted ran for a while, and then recompression tested, it passed.

so yeah i'd do the water decarbon thing, change the plugs/inspect the ignition, and then retest. the good thing is that the water decorboning means less scraping if you do have to pull the engine.

sharingan 19

iTrader: (16)

I just changed the plugs, so I will swap the old ones back in and do the stream clean procedure.

Also worth noting; when I went to pull the plugs I discovered there were 2 leading plugs in the front housing and 2 trailing plug in the rear housing. I'm guessing it probably didn't make much difference at such a low boost level, and when I checked the plugs they didn't look like they were overheated or run lean.

When you say "check the ignition" What procedures are you referring to?

Attached is a pic of the rear leading plug, the front leading plug was a little darker, but it doesn't want to upload.

beefhole

iTrader: (29)

That makes sense. I mean hey, trailing is in the back, leading is in the front.

SpeedOfLife

iTrader: (16)

No, it just depends on how much risk you're willing to accept. I've done the steam test on my own motors with good and bad results before, though I can't say for sure that when my original motor locked during a startup that it was due to carbon dislodging, it was already a tired motor. If it were me I'd probably do it, but it's an auxiliary vehicle, too.


I suppose, did you see your mileage jump during the same types of driving? I was under the impression you'd just done some extra highway driving, maybe I was mistaken.

That definitely sounds strange...

A vacuum leak can make it run poorly, but shouldn't affect a compression test. You had the throttle open during the test?

RotaryRocket8

Haha. I WAS waiting for either you or "Foothill" to jump on that. =P

FelixIsGod29X

iTrader: (50)

The water treatment is similar to the seafoam treatment. I never heard of disaster from doing either one. But its your engine not mine so good luck!

sharingan 19

iTrader: (16)

Yeah, of course. Either that out some kind of Trinidadian carburated rotary drag race trick.

Well, this is my daily, so the stakes are high. However I haven't heard of many engines locking up as a result of steam cleaning. I would like to get back to full compression if possible, but bricking my engine is not an option.

No, I've driven to Tampa many times before...18's (until cool weather, then 17's). Drive around town 17's -18's. This most recent trip was no different, 17.42 on the way down. Filled up before the fun run , tore *** through the night, and made a trip to a friend's house. Filled up to head back to Jax and discovered 19mpg. After I drove back to Jax then went back and forth to work a few times I filled up I'm preparation to head back to Tampa...20.6



Yeah, I was hoping for a vac leak, but I pressure tested from the TID back and couldn't hear/feel any. Yes pedal was to the floor. Although I apparently used the wrong spark plug location

SpeedOfLife

iTrader: (16)

I don't know why you would use the trailing plug hole, its opening to the plug hole is much smaller, slowing the pressure change in the plug hole.

This builder recommends checking from the leading:
Rotary Resurrection home of the budget rebuild.

driftxsequence

iTrader: (8)

Your electrode is all rounded off. It should be flat and not rounded like a ball. That is what I was told by Jim at JPR at least . For stock power levels I'm not sure how much of a difference it makes. Here is an image of a new one for reference.

sharingan 19

iTrader: (16)

Not sure why the trailing location would be better. Where's one of those fsm scans when u need them?

Lol, very familiar with what good condition spark plugs look like. Those were replaced, running 4 trailing plugs now.

Black Knight RX7 FC3S

iTrader: (7)

60-60-30 on the rear is really really low.
Either the seals are stuck somehow, or it can be totally trashed by something breaking off and scraping along the rotor and housing. I would take off the turbo and manifolds, get a flashlight and look into the exhaust port. If the exhaust diffusers in the housing are still in there, it might be hard to stick your finger in and feeling the housing surface and the apex seals.

bumpstart

trailing plug hole give highest result,, esp. in rx7 12a's ..LDR rotor chambers multiply the effect

ive been doing it 20 + yrs.. dont need to read up on it..
proper test is/ has always been via the trailing plug hole
,, if you wish to know why
then the answer is found in a simple observation of some of those mazda scientific papers that also show when and where maximum chamber pressure is achieved

again,, whatever the test is ,, its relative to location,, test rpm.. throttle restriction , engine temp and chamber dryness

bumpstart

http://wright-here.net/files/manuals.../_C_engine.pdf
c-9 .. clearly states trailing plugs ( mazda FSM )
differences are not huge in MDR chamber engines. ... but can be fairly significant differences in the LDR 12a engines
..again it doesnt matter as long as apples are always compared to apples

sharingan 19

iTrader: (16)

Thank you very much! Great info! I doubt the rear rotor will jump up to 90, and I did test the front and rear the same way, but after I stream clean I will use the trailing plug location, they c are easier to get to anyway.

Not sure if this matters, but that Christmas eve run was the first time I ran the engine hard since it was rebuilt 8 months ago. Seems like an odd coincidence that vac would drop and mpg would go up immediately after....

Ive red lined it before and boosted, but never got more than a single gear pull. That night I routinely topped out 2nd and 3rd and started pulling in 4th. Could something have "shifted" internally as a result?


Yeah, I keep hearing that, but I also keep hearing it shouldn't idle at smoothly@ 800 (like it did prior) and hold steady vac (16 ish) so I'm a little puzzled. I really don't feel like removing the turbo to check because:

A) you can't inspect side seals from the exhaust ports
B) it's a time consuming mess
C) I don't have the money for a rebuild, so regardless of what I find I'm going to have to put it back on and drive too work the next day......or walk, depending on how the cleaning goes

sharingan 19

iTrader: (16)

So I decided to try the stream cleaning trick.....

I used the "one rotor at a time methodology" figuring I would do the rear first, and that would make the front easier once it was un-stuck...no dice.

First thing I noticed was the vac nipple pictured below, when I went to put the line on that nipple. It appeared fine until I rotated it to remove it, then I saw that it was trashed.

I proceeded to hook up the line, rev the engine to 2500 and run 750ml of water through, without incident, it smoked a bit, and steamed consistently but all was well. Until I removed the line and it stalled.

I was able to get it started and rigged the pedal to 2500 and proceeded to hook the line up to the top most nipple on the front side of the uim (which normally goes to my bov). As soon as I stated feeding it water, it began struggling and shortly died. This time refusing to start until I used the fuel cut switch.

Now it won't even idle with everything hooked back up, so I figured I'd come type this up.
Perhaps the rear spark plugs a are covered in carbon and unable to sustain the cleaning of the front rotor?

New theory: perhaps that nipple being broken caused a lean condition that did actual damage to the rear rotor?