Need Help

[project 92z]

I have a 91 Z34. My mistress as my wife says. She's sick and I need help from this panel of Drs. The symptoms are mostly barely noticeable except by me. They are some loss of power throughout entire range. 1 full second on 0-60 and top speed struggles to get above 110. Running a bit rich with idle and low rpm being shaky. Start up growl is not as strong (loud). No audible vac leaks and cylinder compression is all close (52 to 60psi).

Recent work includes (all since issues began) new TPS, MAP sensor, coil packs and module, plugs and wires, cold air filter and trans and motor mounts.

I want yup be ad thorough ad possible so I'm also gonna mention that shortly before this started I added about 3 gallons of race fuel tip an otherwise full tank but norm is regular or midgrade gas. The mileage on the engine is unknown cause 3 years ago it was swapped out.

[Psych0matt]

top speed struggles to get above 110. .

I'm not sure it should anyway

shortly before this started I added about 3 gallons of race fuel.

Why?

Running a bit rich. .

You don't say... ^

[Galaxie500XL]

Well...one at a time...

 

The ECM limits top speed on W-bodies to 108, so that's not unusual at all.

 

Running rich at idle...possible O2 sensor issues, they tend to get lazy and report lean when they get old...but I think you actually told us what the problem is without realizing you did.

 

You reported the results of the compression test as "52 to 60 PSI"

 

Compression ratio of an LQ1 is 9.5:1, so doing a little math:

 

Standard air pressure at sea level is 14.7 PSI.  Simply multiplying by this figure by 9.5 (the compression ratio) gives a figure of 139.65 PSI.

 

Now...you're not likely to see that exact number, as sealing isn't perfect even in a new engine, and you might not necessarily be at sea level....but, even assuming 85% volumetric efficiency, you should see a number around 118 PSI.

 

Even in the BEST case, (60 PSI), you're running about HALF of the cylinder pressure you'd expect if everything was right--effectively, you're running around 5.25:1 effective compression.  I'd be concerned if cylinder pressure was much below 100 PSI, and you're way below that.

 

3 possibilities I can think of, either one of them, or a combination of all 3...

 

Severely worn rings

 

Cam timing off significantly

 

Burned valves

 

You didn't mention oil consumption, but it it's high, I'd say rings are a likely suspect.  Burned valves?  Not so much, as ALL of them show low compression, and it's not likely every cylinder has a burned valve.

 

Cam timing, assuming all else is correct is a distinct possibility.  I'd inspect the cam belt, then remove the front valve cover, turn the engine to TDC, and see if the flats on the front cams are parallel with the deck surface of the cam carrier.  If they're not, a timing belt is likely in your future---but if the rings are shot, you've got a much larger problem what will have to be addressed.

[55trucker]

2nd the statement on your cam timing.

 

60lbs?.......how did you do the compression test?

 

my engine at 80,000mile is still at 160lbs,

 

What's the mileage on the engine & has the cam timing belt ever been replaced?

[Schurkey]

Running a bit rich with idle and low rpm being shaky. Start up growl is not as strong (loud). No audible vac leaks and cylinder compression is all close (52 to 60psi).

Recent work includes (all since issues began) new TPS, MAP sensor, coil packs and module, plugs and wires, cold air filter and trans and motor mounts.

I want yup be ad thorough ad possible so I'm also gonna mention that shortly before this started I added about 3 gallons of race fuel tip an otherwise full tank but norm is regular or midgrade gas. The mileage on the engine is unknown cause 3 years ago it was swapped out.

1.  HOW do you know it's "a bit rich"?  Did you test O2 sensor voltage?  Did you use an exhaust analyzer to look for carbon monoxide (CO)?  If you're going off of SMELL, you could be WAY WRONG.  A misfire will "smell rich" even if it's a misfire from being lean.

 

2.  Most vacuum leaks are not audible.  What is the O2 sensor voltage?  What are the fuel trim numbers--short- and long-term?

 

3.  As said, cylinder compression of 50--60 indicates a broken compression tester gauge, or you didn't open the throttle, or the engine cranks REALLY slow, or the engine is broken.  Compression pressure varies with barometric pressure (altitude) but yeah, 150+ would be typical.  I've seen cylinders run with 80 psi compression.  I've seen them run with 50 psi compression once revved-up.  They're so weak at idle that ff all six were that bad, I don't think the engine would start.  I suspect the engine would run so bad you'd get beaten in a race...by pedestrians.

 

I test my compression testers by connecting them to my air compressor using a brass tee and Milton "M" series (Industrial Interchange) quick-coupler plugs.  If the two gauges and the gauge on the air regulator are in reasonable agreement...I can be  sure the compression testers are OK.  A single compression tester could be checked against the gauge on the regulator without using a tee.

 

 

 

4.  "Cold air filter"???  You wasted a bunch of money.

 

5.  Was this "race gas" leaded, and therefore poisoned the O2 sensor?  How old is the O2 sensor?

 

You can screw around with this, you can replace a bunch of parts and hope you get lucky.  Until you confirm the compression test, you're just guessing.  IF (big IF) the compression test was done improperly, and all six cylinders really do have proper compression, you need to connect a scan tool and look at the data stream, and you need to verify fuel pressure.

[Schurkey]

 

You reported the results of the compression test as "52 to 60 PSI"

 

Compression ratio of an LQ1 is 9.5:1, so doing a little math:

 

Standard air pressure at sea level is 14.7 PSI.  Simply multiplying by this figure by 9.5 (the compression ratio) gives a figure of 139.65 PSI.

 

Now...you're not likely to see that exact number, as sealing isn't perfect even in a new engine, and you might not necessarily be at sea level....but, even assuming 85% volumetric efficiency, you should see a number around 118 PSI.

 

Even in the BEST case, (60 PSI), you're running about HALF of the cylinder pressure you'd expect if everything was right--effectively, you're running around 5.25:1 effective compression.  I'd be concerned if cylinder pressure was much below 100 PSI, and you're way below that.

 

3 possibilities I can think of, either one of them, or a combination of all 3...

 

Severely worn rings

 

Cam timing off significantly

 

Burned valves

 

You didn't mention oil consumption, but it it's high, I'd say rings are a likely suspect.  Burned valves?  Not so much, as ALL of them show low compression, and it's not likely every cylinder has a burned valve.

 

Cam timing, assuming all else is correct is a distinct possibility.  I'd inspect the cam belt, then remove the front valve cover, turn the engine to TDC, and see if the flats on the front cams are parallel with the deck surface of the cam carrier.  If they're not, a timing belt is likely in your future---but if the rings are shot, you've got a much larger problem what will have to be addressed.

1.  While your math works out, actual compression is likely to be higher.  Partly because compressed air is heated, and being heated, it wants to "expand".  Since it can't expand, the pressure goes up a little.

 

2.  The four LQ1 heads I've pulled apart all had uniformly horrible exhaust valves.  I was astounded at how bad they were, and how EVERY exhaust valve looked the same.  Even though they're rough as hell on the sealing surface, they clean up with about a .003--.004 cut.  Wouldn't be hard to imagine that all 12 exhaust valves are so wiped-out they hardly hold pressure.  For the record, the hardened exhaust SEATS looked beautiful--I only had to touch-up two of them, the rest were fine.

 

A cylinder leakdown test would confirm--with the valves closed on the cylinder being tested, and that cylinder pressurized with "shop air", air escaping from the tailpipe would signify an exhaust valve leak.  Air escaping from the throttle body indicates an intake valve leak.  Air escaping from the crankcase (remove the oil fill cap) indicates failed rings.  Air escaping from the radiator (remove the fill cap) indicated failed head gasket, or cracked casting(s).

[Imp558]

Compression tests are cool but leakdown is superior. Seeing as how you can narrow the timeframe to the race gas, an O2 sensor seems likely but we're also talking about 100 mph runs so burned rings and stretched belt all do seem just as plausible. Start testing as Schurkey laid out and you'll find it.

 

Well...one at a time...

 

The ECM limits top speed on W-bodies to 108, so that's not unusual at all.

 

Running rich at idle...possible O2 sensor issues, they tend to get lazy and report lean when they get old...but I think you actually told us what the problem is without realizing you did.

 

You reported the results of the compression test as "52 to 60 PSI"

 

Compression ratio of an LQ1 is 9.5:1, so doing a little math:

 

Standard air pressure at sea level is 14.7 PSI. Simply multiplying by this figure by 9.5 (the compression ratio) gives a figure of 139.65 PSI.

 

Now...you're not likely to see that exact number, as sealing isn't perfect even in a new engine, and you might not necessarily be at sea level....but, even assuming 85% volumetric efficiency, you should see a number around 118 PSI.

 

Even in the BEST case, (60 PSI), you're running about HALF of the cylinder pressure you'd expect if everything was right--effectively, you're running around 5.25:1 effective compression. I'd be concerned if cylinder pressure was much below 100 PSI, and you're way below that.

 

3 possibilities I can think of, either one of them, or a combination of all 3...

 

Severely worn rings

 

Cam timing off significantly

 

Burned valves

 

You didn't mention oil consumption, but it it's high, I'd say rings are a likely suspect. Burned valves? Not so much, as ALL of them show low compression, and it's not likely every cylinder has a burned valve.

 

Cam timing, assuming all else is correct is a distinct possibility. I'd inspect the cam belt, then remove the front valve cover, turn the engine to TDC, and see if the flats on the front cams are parallel with the deck surface of the cam carrier. If they're not, a timing belt is likely in your future---but if the rings are shot, you've got a much larger problem what will have to be addressed.

Saar? Lol

[Galaxie500XL]

1.  While your math works out, actual compression is likely to be higher.  Partly because compressed air is heated, and being heated, it wants to "expand".  Since it can't expand, the pressure goes up a little.

 

2.  The four LQ1 heads I've pulled apart all had uniformly horrible exhaust valves.  I was astounded at how bad they were, and how EVERY exhaust valve looked the same.  Even though they're rough as hell on the sealing surface, they clean up with about a .003--.004 cut.  Wouldn't be hard to imagine that all 12 exhaust valves are so wiped-out they hardly hold pressure.  For the record, the hardened exhaust SEATS looked beautiful--I only had to touch-up two of them, the rest were fine.

 

A cylinder leakdown test would confirm--with the valves closed on the cylinder being tested, and that cylinder pressurized with "shop air", air escaping from the tailpipe would signify an exhaust valve leak.  Air escaping from the throttle body indicates an intake valve leak.  Air escaping from the crankcase (remove the oil fill cap) indicates failed rings.  Air escaping from the radiator (remove the fill cap) indicated failed head gasket, or cracked casting(s).

 

That's a pretty nasty looking valve!

 

Good point about verifying the compression tester.  I had a cheap one I'd had for years, turned out it was wildly inaccurate.  I finally replaced it earlier this year with a Snap-On tester set with a hard case I found in a pawn shop for $60.00.

[project 92z]

Wow. So much info. Yes, I did the smell rich test so that could be off. The compression test was done with loaner from O'rielys, fuel relay out, 1 cylinder at a time, crank for 6 sec. Repeat. So could be inaccurate but consistent. And cold air was for looks mostly, I know the ecm compensates for that. Oil consumption is very low. I'll add maybe 1/2 quart between changes. O2 sensor hadn't been touch during the 4 years I've had the car so idk its age either. As far as the belt is concerned, I did it a couple of years ago and gotta check but don't think it's 50k yet.