Let's Talk Vacuum

[rjc]

I purchased a vacuum tester today and compared the information from my Explorer and Ranger. The Ranger has a 2.3 liter 4 cylinder, no air and no power steering and a manual transmission. It had a higher average vacuum of about 25mmHg and steady within less than 1mmHg variance. When the throttle was increased the vacuum increased to about 30mmHg and then when released settled down to the same idle vacuum on the 2.3 liter engine. I've had this one from 23K and have changed the oil and filter very regularly.

The Explorer, which I purchased at 94K, has a lower average vacuum of about 23mmHg in the 4.0-liter engine at idle with about 1mmHg variance at idle until the air conditioning compressor kicks on then it drops to about 21mmHg and steady. When I increased the throttle on the 4.0 I got a steady drop in vacuum from 23mmHg to about 19 or 20mmHg and when I let off of the gas I it went back past the 23mm idle average to about 26 or 27mm and then went back to average idle until the compressor kicked in and then it would drop again temporarily each time it cycled. Why is my vacuum decreasing with increasing throttle instead of increasing with increasing throttle? Any suggestions? The engine is running OK right now with gas milage of about 14 to 16 in town at 18 to 20 highway. I don't seem to be burning oil and my engine doesn't smoke. I do seem to have a little bit of a lope at idle but my 2.3 rattles like crazy at idle so I'm not sure how smooth is "normal" for a 4.0. Do you think I'm getting bad valves? This engine has relatively low oil pressure that gives intermittent reading. I also should add my engine wasn't all the way warmed up when it was tested. Thanks for the advice, rc

 

[rjc]

well....

I've done some internet searches and I guess I'm getting some "backpressure" which may indicate plugged catalytic converter. To check, one suggestion I found was to remove an O2 sensor to relieve back pressure and vaccum test to see if that alleviates the problem. Milder symptoms include lower gas milage and rough idle. More sevear no start conditions. I purchased the vehicle from a local banker who drove it short distances to work. Apparently this contributes to destruction of the converter because it doesn't come up to temp. Also, the spark plugs I removed at 94K had a gap of about 60 or 70 thousanths so they were gapped large reducing the efficiency of the engine which can also contribute to plugged catalytic converter. I did notice that although my vehicle passed smog during the transfer of title, the emissions were considerably higher than the previous smogs in the car records. All this is pointing at the CAT but I'd like to know what you guys consider "normal" backpressure for a 4.0 engine as I have also read some initial drop is "normal" with and increase back to baseline. rc

 

[oahrens]

Sorry

RJC,

Sound like you're on the right track. Unfortunatly, I don't have a vucuum tester to compare stats but I'm curious as to what you find out. If that is the original cat, I would definatly replace it.

 

[rjc]

Testers

Well, I was hoping for more feedback from other Ex owners. I priced the 2 catalytic converters that my vehicle has at $109 each installed. I'm not spending anything on this vehicle until I find out what's "normal" as my engine seems to be running OK just not great and not with the best gas milage. As far as I know this might be normal with the EGR or some other valve opening at a certain RMP. The tester was about $20 at Kragen and I can say with certainty that I will not buy another car without at least doing a quick vacuum test. The Exs have a nice capped vacuum port right next to the master cylinder. This test cannot always tell you exactly what's wrong but it can hint at possible patterns of failure particularly if the needle is jumping a lot or the reading particularly low. rc

 

[Braden D]

rjc, I've also heard about a clogged Cat causing a drop in vacuum. As for comparing it to another vehicle I'll try and do mine but it won't be until this weekend since my time is completely booked until then.

 

[rjc]

That would be great!

I'd sure like to find out if other Explorers do the same since there may be an electronically activated component like EGR or something that reduces vacuum at higher RPMs to reduce fuel intake. I have also read there is a Govonor on the 4.0 that won't let it go past 4,000 rmp to prevent engine damage. I look forward to hearing what your vehicle does. Thanks, rc

 

[mrboyle]

Your engine vacuum levels sound normal. If you are worried about excessive backpressure (due to plugged cat) I'd recomend taking your X in and having a backpressure test done on the cats. This will deermine the condition of the cats far greater than a vacuum tester will.