I have a problem with boost/vacuum readings. I have two gauges, one positive pressure only analogue type, and another digital vacuum/boost gauge.

The vacuum seems to be a little low, fluctuating from 15-16hg. The response on the digital unit is slow, so it's hard to tell the vibrating pattern. The stock rev gauge needle seems to have some slow minor waves of less than 50rpms fluctuation. Otherwise the idle seems ok and smooth when warm, except some minor popping / running rich without A/P. I set the idle at 700, otherwise it seemed a little bit high (with the A/P connected, and sticking for few seconds to 2000).

Similiar things happen in boost. I get very quick build up and by 2500 I get full boost of 9psi (wg full opened), but the boost reading fluctuates around 8-10 on the digital gauge. The analogue gauge shows the same boost, just seems to vibrate half less than digital gauge, around 8,5-9.5psi.

What could cause this behaviour?


I'm suspecting leak after the TB. If I leave the nipple on UIM uncapped, the idle gets a bit more fluctuating and goes up for 100-150rpm. So if I have another similiar leak somewhere, this would probably make sense. I will try to pressure test the system in a few days. If I don't succeed, I will just use the carb cleaner :-/

Few days back, I've also figured that my bov start leaking 6-7psi and crv around 5psi. Altough, I don't believe this could be the cause, at least not for vacuum needle shaking - because it's before the TB. I have a large transition dip, I think this is what crv causes... Otherwise, I'm surprised I get this much boost event with leaky bov, is this even possible?

Help is appreciated.


I have forgot to mention the state of the car: intake, dp, mp, catback, a/p disabled, pettit ecu, new plugs, new wires, 900mile new (not rebuilt) engine.

 

Ported motor?

Also, does the car have good power? Seems to boost too quickly, could be a problem with your spark plugs not all firing.

 

If you are running a Pettit ECU and no Air Pump, your car is going to be lumpy trying to idle around 700-800 rpms (i.e. those fluctuations you are seeing). You'll have to raise it up to about 1000 rpms to make it smooth without the air pump.

 

Mahjik, the 'lopey' idle is due to the rich idle condition running without an airpump causes right? Could the piggybacks compensate for this or are we limited to stand-alones to resolve that issue?

Thanks,
~Kris

 

I guess that would depend on the adjustability of the piggyback. I'm not familiar with the various piggybacks and what they can do so I can't give you a definite answer.

Honestly, inceasing the idle to 1000 rpms from 800 rpms is not a deal.

A full programmable ECU will take care of the problem.

 

GoodfellaFD3S:
The engine has stock ports. Power is good. I'm not pushing it since it still low on mileage (and I need to change oil), but I do kinda have a feeling about boost pattern I get. I get boost creep (full exhaust, ported WG, no pills in the WG line) on primary up to 9psi and on secondary about 11,5 and probably up (I'm not trying it). I will probably have to install restrictor plates :-/ But that's another issue.
How can I check, if all plugs are firing? Power delivery on the primary turbine is smooth. Also, is there a connection between quick spool up and plugs not firing? I have the 99 twins, hence the quick spool up.


Mahjik:
I have the airpump installed, but it's not connected. Neither electrical connector to keep it running neither the pipe to the midpipe. I plan on getting some resistors to replace it and fool the ecu. It seem that it is more responsive in the lower revs with the air pump hooked up and the idle sits 200-300 rpms higher.

Right now my idle is fluctuating around 750 +/-25, so it's not that lopey. Altough, the
vacuum changes for about +/- 2hg. Is this normal, the vacuum to change so much at this fluactuation of revs (+/-25 revs)?

I will try setting it around 1000rpms and report if it still fluctuates. Tough, I really like 700rpm idle - it so sooo much quieter than idle at 1000rpms.

It probably running rich - misfiring under 2000, it seems to do it more if it idles for a while. By misfiring I mean I get some small poping out of exhaust, and the rear bumper is getting black already. (I also get backfire (only sounds, no visual fire or anything) during shifts or I let off the gas...)


And now, back to the topic of shaking boost needle. Today I repeated the test, and it seems it's shaking less, and only after 7psi of boost. So, can I contribute this to leaky BOV - it starts leaking at 7psi?

I also did some leak testing... I have a question tough: Should the vacuum stay in the intake manifold after I shut the engine down? For example, at the idle I have certain amount of vacuum. I shut it off... the vacuum resets to atmospheric pressure. How does it releases, trough the exhaust ports (sorry if it's a stupid question)?
Because, I tried pressuring the UIM trough the nipple with up to 15psi. The pressure doesn't stay in there and vents somewhere out. Where does it vent out (exhaust ports?)? I didn't find any hissing under the hood tough, while doing this.

 

It's been talked about before and the consensus is that the resistor will make no difference. You will be missing additional air which the air pump was supplying. The way to fix that is by adjusting your fuel usage at idle/low rpms which you can't do unless you have a programmable ECU (or maybe a piggyback ECU).

If your idle rpms changes, it will change your vacuum reading. So if your rpms are bouncing up and down, your vacuum will most likely be bouncing with it.

 

Actually, I never had the air pump physically connected to the midpipe. I was only connecting/disconnecting the electrical connector. The difference for me is obvious:
- about 250 higher idle
And the air pump was pushing air next to midpipe, not INTO the midpipe. Also, it maybe was a bit better response in lower revs and maybe a little more popping from the exhaust around 1500 rpms. Also, sometimes the idle was sticking for few seconds at 2000 rpms. It never happened to me without the air pump connected. I might be other issues that result in such behaviour and also since changes weren't it's hard to tell if they are present by the feel. So I'm not completely positive these last observation were reactions by air pump connected, but the idle definately got higher for appr.250 rpms - I tried it several times, when I (dis)connected the air pump.


I do understand that, but does the relation seems ok? 2hg differnce for 25rpms or so?


What do you think about my pressure testing the UIM? Should it hold the vacuum or boost after it is shutdown, or is the pressure suppose to escape trough exhaust ports? I would really appreciate explanation.

 

It's not the air being connected to the midpipe per say that causes this problem, it's the secondary air injection through the ACV which is attached to the LIM where the problem is.

If the UIM held vacuum/pressure after shutoff, you would be able to register it on your vacuum boost gauge. But while the engine is off, the boost gauges are sitting at 0. There may be a little pressure in there, but it should be much if any.