Issues with low vacuum

[platez808]

My car starts up, idles, and runs fine when cold; but as it warms up, the vacuum at idle drops to about 12 in/Hg from the typical 18-20in/Hg and it gradually runs crappier and crappier, which includes hunting idle and stumbling. This just happened randomly one day about two months ago and am finally giving up trying to figure it out on my own.

I have a series 2 RB25DET in an s13 hatch. It has wiring specialties premade harness, isis intake mani with q45 TB, FMIC, apexi mushroom filter, stock turbo/mani, splitfire coilpacks, ngk BCPR7es spark plugs, 3in exhaust, oil catch can, greddy type RS recirculated, and redid all the gaskets/oil pump/water pump/timing components about a year ago . I have both the stock and a z32 nistune ecu with stock maps as well as the stock maf and a z32 maf, which I've tried multiple combinations of and have had no luck.

I've QUADRUPLE checked for vac leaks by spraying around with starter fluid, replacing all hoses, and redoing my intercooler piping and I feel like there is no way I have a vac leak. I've checked my timing both mechanical and base timing of the CAS. I've checked the TPS voltage. Everything seems to check out and I still have the same issues.

The only thing I can think of and haven't tried yet is a compression test but I'm trying to make time to rent one and do it. Is there anything else I should try??

[RaNMaN]

have you checked your iacv?

i much prefer the boost pressure testing method using and compressor and soapy water over using lighter fluid or brake cleaner which could be damaging the rubber seals. you might want to give that a try instead.

[platez808]

I forgot to put that I actually deleted the IACV and use the thermostatic idle control built-in to the q45 throttle body, which, once it was dialed in, worked way better than my IACV ever did.

And I was using starter fluid (the kind used to help turn motors over) and not lighter fluid. I was thinking of trying to do a boost pressure test but right now I don't have the time to really fab up something for that. I'll look into it though. Thanks for the suggestion

[rcdad123]

i`m not familiar with RB engines, but if they have a pcv valve, check that.

[platez808]

i checked the pcv valve and it seems to work fine but i might just replace it anyway since its so cheap. thanks

[rioredstang]

When it stalls out does it foul plugs? Usually when I see manifold vacuum drop it's a mechanical problem. Did you pull valve cover and see if anything is moving?

[platez808]

When it stalls out does it foul plugs? Usually when I see manifold vacuum drop it's a mechanical problem. Did you pull valve cover and see if anything is moving?

No it doesn't foul the plugs. I didn't pull the valve covers yet but I did check the mechanical timing by checking how the cam and crank pulleys line up and it looks perfect. But I am definitely afraid that it might be a mechanical issue somewhere else as well. I'll take a look under the valve covers. Thanks

[Dro]

Low vacuum may be caused by badly worn piston rings, being that they don't seal well enough to pull the full amount of vacuum needed. Rent a compression tester so you can do a running compression test as well as a normal test.

If you have a vacuum pump you could put vacuum on your whole system and if there is a leak in a hose/tube or possibly gasket you will see the reading decrease over time.

Another thought, it's possible that a piston and/or ring is out of shape and seals perfectly fine when cold, but as it starts to expand as the engine warms up it leaks and causes your vacuum to drop.
Get that compression test done and report back!

[platez808]

Low vacuum may be caused by badly worn piston rings, being that they don't seal well enough to pull the full amount of vacuum needed. Rent a compression tester so you can do a running compression test as well as a normal test.

If you have a vacuum pump you could put vacuum on your whole system and if there is a leak in a hose/tube or possibly gasket you will see the reading decrease over time.

Another thought, it's possible that a piston and/or ring is out of shape and seals perfectly fine when cold, but as it starts to expand as the engine warms up it leaks and causes your vacuum to drop.
Get that compression test done and report back!

So I finally got my hands on a compression tester and I got 140 140 130 134 140 145 from cylinders 1-6 respectively with a dry test. I also performed a wet test but I didn't have a way to put consistent amount of oil in each cylinder and I got 162 175 160 180 180 160 so I'm just going to go with the dry test results. The dry compression test results look pretty good right?

[cotbu]

I have a question why is having 20inhg vacuum so important to you?

I know why you'd want good vacuum ideally ,but your first post doesn't even come close to touching those reasons. You can have low vacuum and have the engine idling perfect.

So why not make this thread about the engine running crappy, but providing the low vacuum as information too compliment the shitty running of your engine.

I'm very familiar with nistune, so just swapping the vq map and mafs might not allow the engine to operate like stock, It might require some actual tuning. If you keep going in your current direction your looking at rebuild options only!

So after you've exhausted other options and opinions, hit me up. I'll walk you through what I would do. And if i happen to save you $500+, I would only ask that you donate.:kiss:

[platez808]

I have a question why is having 20inhg vacuum so important to you?

I know why you'd want good vacuum ideally ,but your first post doesn't even come close to touching those reasons. You can have low vacuum and have the engine idling perfect.

So why not make this thread about the engine running crappy, but providing the low vacuum as information too compliment the shitty running of your engine.

I'm very familiar with nistune, so just swapping the vq map and mafs might not allow the engine to operate like stock, It might require some actual tuning. If you keep going in your current direction your looking at rebuild options only!

So after you've exhausted other options and opinions, hit me up. I'll walk you through what I would do. And if i happen to save you $500+, I would only ask that you donate.:kiss:

Well the main reason I emphasized the vacuum so much is because I really didn't see anything else that changed in my setup when it started running so crappy other than my vacuum not being at the levels they were before. I figure that I'd try making that the title of the thread because that's the most specific "problem" I can come up with and that maybe people who had similar issues before would be more drawn to it, rather than being just another "running crappy" thread.

I know that running a nistune ecu requires tuning, but I just put that information out there to show that it runs equally crappy with both my nistune ecu and stock ecu to rule out my ecu being a possible issue. Right now, I'm running the stock ecu and stock maf to try while I troubleshoot the issue.

What do you suggest that I do?

[cotbu]

Check the ecu for codes.
I have an issue when people say the checked timing and it's spot on or something to that effect. What is base timing and base idle, degrees and RPM. CAS position is only important when we know the actual timing. You can then shoot the crank and tell us the normal idle time after warm up. Tps voltage was? fuel pressure? Also air/fuel ratio?

Have you check the boost gauge signal line for blockage or leaks?
I probably would have spent the weekend ripping apart my car, if i needed or wanted it running for Monday.