Engine rebuild.... low vacuum at idle
 

i rebuilt the motor due to a blown apex seal... got it all back together and i only have 12" vacuum at idle. at 2000rpm it pulls 20"... due i have a vacuum leak? i checked what vacuum lines i can see and found nothing. i did a compression test to just make sure and i have good compression... what should i do? also the car has a little trouble starting... cranks and cranks then sputters and slowly goes up to 1500rpm and then drops to 850... could this just be the relearning procoess of the ecu? i have pfc... any info would be great

If it's a fresh rebuild it's normal for compression to be low for the first 1000 miles or so.

can anyone else back up this statement?

condition of housings? ported? 3mm?

If you have a steady 850rpm idle and vacuum is only 12, then I hope it is street ported. As Allrotor is hinting towards it could be high mileage housings, 3mm seals or some 2mm seals dont seal as well as others. Either way, even a decent sized street port should pull 12 or more vacuum after 10-15 mins of run time.

I wouldn't get too excited over vacuum readings after initial startup. Put some miles on the motor and see how it does.

Dale

3mm seals lower vacuum numbers?

I didn't know that, I have a stock port motor on 3mm seals and have always wondered why my vacuum is 1-2in. lower than most

They just typically dont make as good of compression as 2mm seals. Vacuum readings can be effected by a lot of different things so nothing to get caught up over. If it starts quick when warm, then nothing to worry about.

3mm seals do not generally seal as good

I've used Mazda OEM 3mm seals in my last rebuild. 20k miles on the motor and and compression averaged 108-110 on all sides using a rotary-specific compression tester, both rotors...

housings are in good condition only have about 20k miles on them. stock port and 2mm 2 piece seals from rotaryaviation.com.... put 40 miles on car tonight and now at 13" vacuum at 800rpm and 16" at 1000rpm... besides for the low vacuum at idle car drives perfect

Sounds normal for the aviation seals and the motor probably isnt super tight. The aviation seals always take a little more time to make a good seal. You will be fine, get it on the interstate or nice back road and keep it up at 3-3500 rpms for a 3-4 hours and it will get better quickly.

THat was my seal of choice before the ALS seals came out. Compression usually continues to build up to 20-25k miles. But 108-110 isnt exactly stellar either. Still a nice solid motor though. And the 3mm OEM seals hold up great and dont kill the housings.

good to hear thanks for the info... any idea why it starts so rough? cranks and cranks then sputters at like 500rpm for 10 sec then settles in at 800

Im pulling 12 on my large streetport, less than 800 miles, als seals, and less than 10k housings I bought from DJSeven. Starts very quick when warm. Also 6000 Ft above sea level. Tuning affects vacuum too.

Compression varies from all sorts of factors. If you have good compression and the car idkles fine don't sweat it. It will come up especially if you used old housings.

update: its been 400 miles since the rebuild and vacuum at idle has come up to 15".... on the pfc commander the vacuum at idle shows about 450mm when converted to inches is 17.7... the pfc should be dead accurate correct? so my gauge is just off

Subscribed. Thanks for the info on vacumm reading and how different motor setups affect them.

Aviation seals need more seating time than the others. It's also a great idea to spray carb cleaner around the intake flanges and such to check for minor vacuum leaks. This should always be done after a rebuild or when the intake parts are removed. If a leak is present, the engine will hesitate/stumble. Intake leaks will also make it harder to start as the ecu will see lower vacuum reading and start injecting more fuel based on a lower vacuum reading. I don't care how tight you build your engine. If you have small intake leaks, you will never get high idle vacuum readings.

the pfc should be dead accurate correct? according to the commander my vacuum is about 18" but my gauge only reads 15"... where does the ecu get its vacuum reading from?

The PFC uses the stock wiring, so the reading will come from the factory map sensor just behind the UIM on the firewall. Most hook up their after market boost/vacuum gauges to either unused nipple on the passenger side of the UIM. When you do this, you will mostly be getting the reading of your individual rotor (front or rear depending on nipple). So it's very possible to get different vacumm "gauge" readings from each nipple depending on how strong the compression is from the individual rotors. The map sensor gets is readings just after the primary throttle plates of the throttle body. In this location the primary runners are just starting to split into two separate runners so the map sensor is actually reading the accumulated vacuum of the both front and rear rotors. You can try to "T" into the map sensor vacuum port with your guage to see if the vacuum readings start to match. If they don't try a different gauge.

If you got 450mmhg of vacum at idle let's say 850 rpm you got a realy good sealed engine.

With streetported engines and 2mm seals im realy happy if it's at 370+mmhg on startup. This is with used housings.

But 450mmhg is as good as i have seen of the 150+ engines i have rebuilt. So should be good for a while.

Do you still have problems starting it? If so check your startup map. It might get too much fuel when trying to start or to little. So just fiddle abit with the startup map and it should most likely get better :)

JT

k thanks a lot for the feedback... and to jantore i do still have problems starting it but just once in a while which is weird. i really wouldnt know what to adjust using the pfc... ive messed around with hondata with my rsx but not familiar with the system on the fd... any decent tuners in the northeast (MASS) area?

and also could i leave the boost gauge t'd off the map sensor or would that mess up the map sensor?

Check out this thread for starting issues with pfc:

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/hard-start-long-start-discussion-modified-setups-897249/