86 951 Driver

My vacuum gauge is always pegged. I have just read that thread about the orings on the oil filler neck. Could it cause this condition. My car doesn't leak any oil on the ground but does burn some. Also whenever I shift a cloud of smoke comes out the exhaust pipe. It is blackish color. I think it could be all related. Also my car is a 1987 924S.

adrial

What is the lowest vacum that your gauge will read?

Black smoke out the exhaust = fuel, which means you're a bit rich.

How is your oxygen sensor? When was it last replaced? If you unplug the sensor, does the car run better/worse/same?

Have you ever done a compression or leakdown test? This would be a good idea as it will give you a very good idea of the condition of the top end of the engine.

Yabo

fHey wher edo you get a vacuum gauge, and where did you install it? or is it stock on your car? I alwasy wanted an easy way to check the vacuum.

86 951 Driver

I have a stock vacuum gauge. It reads below 20 MPG all the time whenever I'm on the gas. I have 175-180 psi of compression on all the cylinders. I just replaced the 02 sensor. That did make it run a little better.

adrial

Thats not a vacum gauge, thats giving you an instantaneous reading of gas mileage.. MPG ... miles per gallon.

Vacum is measured in Inches of Mercury (In. Hg)

Search the archives for more info on that gauge.

86 951 Driver

I searched the archives and is seems that when on the highway it shouldn't be pegged like mine is. I am trying to determine if the FPR is going south or if the AFM is need of a tune.

eman930

Here's the thing the MPG gauge is not very accurate and too many thing effect it. If you wanna know vaccum then hook up a vaccum gauge to the intake. WHats your fuel pressure running at???

86 951 Driver

I don't know what my fuel pressure is. I don't feel like spending $60 plus on a fuel pressure gauge.

Yabo

Well, if you dont feel like spenidng money on a gauge, quit trying to diagnose your problem. simple as that.

xsboost90

could just be an inaccurate guage.....not a vac guage though.

Bart

Why is that?

Charlotte944

Neon:

The MPG gage is more of an EPA placebo than it is a qualitative measuring device.

The major input to this gage is injector duty cycle, or how long the injectors are open. However, as was previously mentioned, there are several other engine parameters that will have an affect on this gage, like vacuum and fuel pressure, because these parameters have a direct impact on the fuel/air mixture which is also related to injector duty cycle.

Before we can do any serious troubleshooting we need to know some of the car's history.

For example, how long have you had the car, has the gage always been pegged, what repairs/modifications have been done, and so forth.

In the mean time I highly recommend using a vacuum gage to check vacuum at the brake booster and at the FPR. Once we have some numbers to crunch maybe we can figure out what is going on.

FSAEracer03

Well though the MPG gauge isn't a very accurate gauge... he's right. It is a vacuum gauge. It's a vacuum gauge that is ballpark guessing how much fuel going into your engine with relation to wheel speed all by means of intake vacuum (showing throttle position). I think the gauge just might be misleading, however.

As for the black smoke when you shift, that does sound like a vacuum leak in the intake system. When you shift, your AFM isn't getting the signal that air is still passing through. The DME gets word that the throttle is still ope and has the injectors pour some more fuel in there resulting in richness unitl you are back on the gas. If this problem happened at the same time you noticed the gauge went whacko, they are related, and I don't think they'll be too hard to diagnose.

There are two vacuum lines connected to your TPS, one before the throttle plate and one after it. The one before heads to the rear of the engine bay for A/C and emissions stuff. The one after is the full-throttle signal for the FPR and stabilizer. Be sure to check all three lines that are involved with the full-throttle signal as well as the 3 connectors and 1 Y-splitter. Also look the the AFM to make sure it's not loosing vacuum at it's connections points with the intake. Double check the electrical connector. There are two vacuum points underneath the intake manifold, and though I don't think they are the cause, as they usually make the engine not want to run, check them as well. The front point right runs to the idle stabilizer and J-boot, the rear runs to the oil inlet ni the crankcase.

I think you're on the right track with the gauge oddity and black puffs being related to vacuum. Best of luck tracking it down!

86 951 Driver

Ok here is my whole school of thought with the fuel pressure gauge is that I can have a shop look at it for an hour for $60 or buy the gauge. I am not a very rich person. I have been working on this car for 3 months. When me any my dad first got it. I have cleaned the fuel injectors manually. I have changed the spark plugs, o2 sensor, and put a new exhaust on it. The car didn't have an air filter or fuel filter when we bought(reason why the injectors were clogged) it but I have put that stuff on and it runs a lot better then it did 3 months ago. The running rich thing concerns me because my MPG is always pegged. It has always been this way ever since were first got it. I started looking at the Ignition system and the fuel system. I can't tell if the AFM is bad. I believe the TPS is good because it does click. When I unplug the afm the car can barely idle and it can't go above idle. I have read a lot about the FPR so I was thinking maybe this is my problem, or there is a vacuum leak. I read about the orings around the oil fill tube. I read that it can cause a vacuum leak, and make the car ingest oil in the intake. Sorry for the long post.

eman930

Neon, I think your problems is a bad vaccum to the MPG gauge or the gauge itself. You need to start looking at the small simple things before you start looking into the big stuff