I just checked the vacuum pressure on my truck. The needle on the vacuum gauge bounces wildly between 17 and 18 Hg. When I rev the engine it pretty much stays in the same place, maybe a slight increase but that needle still shakes wildy. When I let off the throttle it breifly spikes up to 20 then settles back to shaking between 17 and 18. Is this normal? I did find a bit of an air leak around the brake booster check valves' grommet. I've never tested vacuum before. The exhaust is still popping by the way, she's not happy.

 

Sorry to waste bandwith but I just tested the EGR valve and it's vacuum controller with a vacuum pump. There doesn't seem to be any vacuum being applied to the valve. I hooked a vac gauge to the line that goes from the controller to the valve and from what I read I was supposed to see some vacuum applied, there was none. I also tried applying vacuum to the EGR valve with the pump and did not notice any change in engine rpms. Did I just do something wrong, or are those components likely to be bad. I did some electrical tests on the EGR controls and they seemed to check out ok.

 

I read that the vacuum fluctuations could be caused by a valve problem and that I should check compression. I did the test, the results make me wonder if I did it right but I followed the instructions. The results were:

Cyl 1- 65
Cyl 2- 75
Cyl 3- 65
Cyl 4- 85

Does this look right? Does it indicate a potential problem, or did I just do the compression test wrong. I tested each cylinder four times before moving on and these are the results I got pretty much every crank. I would hate to think that there could be engine problems, it's a rebuild with only about 30k on it.

 

I didn't look at the specification but that's to low.

Did you have a fully chraged battery, have all the plugs out, have the gas pedal to the floor?

If you did it correctly try putting 5-6 drops of oil in the cylinder right before you run the test, if it comes up them you may have a problem with the rings.

 

I was checking things out again and noticed a potential timing problem. When the cam is on its timing mark and the dist is on no. 1 the crankshaft is at about 4-6 BTC. Could this problem be causing the problems, including the whacky compression numbers? I'm thinking maybe my timing belt jumped a tooth? What would cause that to happen?

 

I wouldn't expect the valves to be open that soon, but if it's off thats a problem.

Can someone give us a second opinion????

 

I've never tested the compression on my range but i've done a few chevy small blocks and they range from 120-160 PSI

 

If the cam is on the timing mark, the valves *should* be closed, as it it Top Dead Center on compressio. The dist pointing to #1 is a 'ballpark' as the rotor will jump the spark to the closest plug wire terminal (we hope..) The crank should be at TDC the same as the cam. I don't think this will cause the problems described, it's too close to being exactly correct. With the cam timing off, or the ignition timing set before the engine has warmed up, you can get an engine with way low torque at low rpms, and will 'come on cam' at a high rpm.. or if set the other way, will have a good low end, but no top end. I chased mine after changing the timing belt for days, until I read the EEC-IV manual that timing would be unmolested by the computer for 2 minutes after extracting the codes, engine running. Woops, I'd set the timing before the computer had firmed up the 'warm' timing setting. Wee-tarded, I'd say. No bottom end, and hardly able to get out of its own way. Felt like a jillion hp engine after setting correct timing...
tom

 

Irishguy, I know you said earlier that you had "adjusted" the distributor, so if you've really cranked it around or have removed it & didn't get it installed propery, the distrbutor position could be off some!!!

How many mles on this timing belt????

How did the MAP & O2 sensor tests go????

You seem to be jumping around from one thing to another, without clear feedback to us, on what you've found on previous checks, so it's difficult to try & follow your troubleshooting logic & know where we are!!!!

On the compression numbers, they sound low to me!!! Did you oil the cylinders & check the other things as Ken suggested & retest compession, if so what were the retest numbers????

 

I'll try to answer your previous question's about your observed vacuum readings & compession readings.

The compression readings should be a minimum of 100 psi & the highest to lowest reading should be within 15 psi of each other.

The low compression readings you posted aren't normal at all.

You pobably didn't do something right, as your posted idle vacuum readings were within the normal idle range of 15-20 inches vacuum, except for the rapid vibration.

So if your compression was really that low, you wouldn't have normal idle range vacuum readings. IMHO

If that vibration was as you said, changing rapidly, by ONE inch of vacuum, it would suggest something like a timing problem.

If the reading is rapidly changing by say 4 inches of vacuum, combined with blue exhaust smoke, then it would suggest something like worn valve guides.

If a rapid 4 inch vacuum fluctuation occurs with an INCREASE in engine speed, then it would suggest things like, leaking intake manafold or head gaskets, weak valve springs, burned valves, or ignition misfire.

A normal engine at a steady 2500 rpm will show something like a fairly steady 19-21 inches of vacuum, with no wildly jumping needle.

If you quickly go to wide open throttle, it should drop to nearly 0, then as the engine rpm's rise & you quickly close the throttle, it should jump to 21-27 inches of vacuum, (or jump say 5 inches above the idle reading) then return to it's idle value WITHOUT DELAY.

If the vacuum doesn't peak, say 5 inches or so, above the idle reading & returns SLOWLY to the idle vacuum reading, then it would suggest worn rings.

If there is a long delay in returning to the normal idle vacuum reading, it would suggest a exhaust restriction.

Hope this will shed some meaningful interpretative light on your observations.

 

Thanks for that vacuum explanation pawpaw. Everything happened just as you decribed it should in a healthy engine, except for that little constant fluctuation. Combining that with the fact that the timing is a bit off I think that is the problem I need to correct first. When the cam is on its timing mark the crankshaft is at 6 BTC. I'm thinking maybe the belt jumped a tooth. I'm going to correct the timing and see what difference it makes. If it solves the problem I suppose it would be wise to invest in a new belt and tensioner.

Would the timing being off like this cause the constant lean 'code 41' that the computer is reporting? If not, I'm going to pull the O2 sensor and perform the tests that are decribed in the tech link.

 

IMO, the system lean code 41, is likely caused in your case, by a vacuum leak, or MAP input problem to the computer.
If you hve a input side vacuum leak, it can confuse the MAP & cause it to send the computer bad data, which in turn can mess up he injector squirt time. Dang vicious circle isn't it!!!!

The O2 sensor is just reporting what it sees downstream from the combustion chamber.

The computer takes inputs from various sensors, to know when & for how long to squirt the fuel injectors & when to fire the spark plugs.

Typically it uses the crankshaft sensor to fire the spark plugs & the camshaft sensor to tell the computer WHEN to squirt the fuel injectors.

The clues it gets that help it figure out what to do about the LENGTH of the injector squirt time (& thus the richness of the air/fuel mixture), come from things like the, MAP/MAF, ECT, AIT, TPS, Speed Sensor, tranny & probably some I've left out, but you get the idea.

Anyway, of all those that figure in on the injector squirt time, the only one you have posted a code clue on, is the MAP sensor!!!!!

So once you get the timing thing squared away, I'd look to testing the MAP sensor, it's reference voltage & it's vacuum connection & hoses. If you have ANY input vacuum issues that can confuse the MAP sensor & thus the computer, they need to be put right, before you go off on another tanget & further confuse yourself.

Just go where the codes lead you, concentrate on knocking them down one at a time & sooner or later things will begin to make sence to you & the problem/s will make themselves known!!!! IMHO

 

I'm still plugging away at this, I've adjusted the timing and replaced a faulty EGR position sensor so far. I haven't got her all put back together yet, and I have a question regarding testing the MAP sensor.

This link was provided which gives a testing method for the sensor: http://www.kemparts.com/techtalk/tt11.asp

I have a question about measuring for "Hz". If my DVOM has this capability would it be labeled as "Hz?" If not, what setting would I use to perform that test?

 

FWIW, Hz is the 'sign' for frequency. Named for a smart fellow named Hertz. Your DVOM would have a setting with that label if it had the capability to measure Hz. Generally, a bad MAP sensor will cause a whoopsy idle. Up.. down... wander all over the place. It can also cause the idle to be too rich. (or lean, I guess) It measures vacuum in the intake manifold, and converts this measurement of "load" on the engine into a Hz (frequency) signal. An engine with low load will have higher vacuum than one that is heavily loaded. When you floor the gas, you mostly will have a high load on the engine.. and very little manifold vacuum, as an example. At idle, the vacuum is high and the load is low.
tom

 

Well if your DVOM is the automotive type with a rpm scale, you can do a rough frequency output test with it, to see if the MAP is responding to pressure changes.

Otherwise you'll need a digital device with a waveform graphing display, or an oscilloscope, to actually see & measure the frequency change.

So is your meter a automotive type, with a RPM setting/scale????

If so, at KOEO, with 0 inches of vacuum applied to the MAP, it's output will typically cause a reading of 300-320 RPM.

With 5 inches of vacuum applied, it'll typically read 275-295 RPM

With 20 inches of vacuum 200-215 RPM.

If you don't have a RPM scale on your meter, you'll have to measure the output voltage change of the MAP & see if it's output changes with changes in applied vacuum.

It's voltage to the computer should drop with an increase in appied vacuum. Say from around 5 volts at 0 vacuum to maybe 1.1 volts at 20 inches of vacuum, anyway you should see a voltage drop with vacuum applied.

This by no means is a calibrated good or bad test, just a function test, to see if the MAP is awake & responding to pressure changes. Sort of a working or not working test!!!!

Have you done a THOROUGH inspection of ALL your vacuum lines & connections????

Looking for splits, cracks, kinks, loose fitting connections to ANY vacuum using components. The brake booster is a biggy for instance, as well as the evap canister, so be sure to do a good check of their lines & fittings.

I know I keep harping on this, but it's important, because ANY vacuum leak can cause the MAF to send the wrong information to the computer & it'll begin to mess up the fuel mixture & ignition timing!!!!

Let us know what you find!!