New 5.0L built, low compression and horrible MPG

[1997XLTRollover]

Hey guys, long story short I completely rebuilt the 302 in my 96 Explorer. It's now a 306 and had the heads cleaned up so may have a hair higher than factory compression. Engine is still stock everything including cam and heads, only had a minimal amount taken off to clean the head deck up).

Anyway I have 1600 miles on the engine now and I can't get better than 10-11 mpg. It also sputters once every couple weeks like it wants to stall. I'm thinking it doesn't have the power to keep running because the compression is too low and that may be causing this. I checked for vac leaks and found none at all.

The truck seems to run decent, starts up ok and has no real drivability issues or noises but isn't super super smooth and kind of seems flat (my buddies say I'm crazy but something felt off). So this lead me to do a compression check.

I only tested 3 cylinders, 2 on the passenger bank and 1 on the drivers and all 3 read 125-130 psi cold and 130-135 warm without adding oil to the cylinder. This to me seems pretty low, could my machine shop have screwed up the hone job and my engine isn't broken in right? Or am I freaking out for nothing? I feel like the thing should have more power than it does and have at least a 160 psi cylinder pressure.

 

[bill06447]

Cam timing may be wrong. Who put the chain/gears together, you or the shop?

Bill

 

[koda2000]

that's very low compression. should be around 165-170 PSI. rings or valves are not sealing. did you open the throttle body during your test? did you try squirting a little oil in the cylinder to see if the compression goes up?

 

[pcam13]

Being its brand new... I would venture to think it is one tooth off on timing chain.

 

[Kiliona]

I would also agree with the others, double and triple check your cam timing before anything else. Also... that was 125 psi cranking compression correct? If that were 125 running compression that'd be plenty

 

[1997XLTRollover]

I built the engine. Throttle was wide open during the test, all plugs still in. I timed it and built it all myself and double checked my timing when assembling, but cam timing being off was my first thought. The only thing Is It runs ok, no popping out intake or exhaust. Would it still run and not pop at all if the cam was off a tooth?

Engine was tested while cracking using the starter, not running. My next step was going to be test with oil. The machine shop assembled the heads, not me but I would think if they messed up the valve job my numbers would be further off.

 

[boominXplorer]

I have had similar issues with my 5.0 swap in my ranger. What valve springs are you running? If they are stock they are too weak to close the valves after lapping them (my issue) Also its very easy to misalign the cam pos sensor, I'm still adjusting mine to this day. Its pretty hard to put a small block timing chain on wrong, it's simple to line up and put on.

 

[1997XLTRollover]

I have had similar issues with my 5.0 swap in my ranger. What valve springs are you running? If they are stock they are too weak to close the valves after lapping them (my issue) Also its very easy to misalign the cam pos sensor, I'm still adjusting mine to this day. Its pretty hard to put a small block timing chain on wrong, it's simple to line up and put on.

25% stiffer valve springs than stock, cam sync should be correct, I used the tool to align it, but either way that won't effect compression, just running.

 

[Dono]

Was it a new timing chain and gears? I've heard issues with the alignment dots being off. I can't imagine you degree'd the cam on install to make sure events were correct before install? No way compression should be that low. I'm no expert, but that's the only thing that makes any sense. Sorry. These issues after all that effort really blows.

 

[1997XLTRollover]

Was it a new timing chain and gears? I've heard issues with the alignment dots being off. I can't imagine you degree'd the cam on install to make sure events were correct before install? No way compression should be that low. I'm no expert, but that's the only thing that makes any sense. Sorry. These issues after all that effort really blows.

Yeah it does really suck. Everything was new. I mean everything. Even down to factory Ford lifters and brand new fuel injectors. Only parts reused from the original 143k mile engine were block, heads, rods, crank, valves and rockers. All else was all new.

I Did turn the engine by hand and wait for a pop out the exhaust to make sure it was right, but didn't degree anything. It was a basically stock rebuild. I got weak pops but figured maybe it was because it was a zero mile engine. I lined up the gears and chain. Could the gears and or chain really have been marked wrong? Wouldn't I get popping out the intake or exhaust of the cam was off a tooth?

 

[Dono]

I really don't know. It just oozes cam timing since it sounds like all cylinders will test low. Could you check all cylinders just to be sure? Low compression has to mean cam timing, ring seal, or leaky valves for some reason. If all cylinders read the same level of ****ty, it sure points to cam timing.

 

[Dono]

Maybe the boys over at sbftech would be more qualified to help you. Those guys build our motors all the time.

I'm having a short block built and the cam is coming installed for me and pre-degree'd. How's that for whimping right out? There's just so much that can go wrong.

 

[1997XLTRollover]

I instantly thought cam timing, I'll have to do some more testing. Unfortunately it's my dd so it may take a bit. I'll try to get some oil in a cylinder and test all 8.

 

[SoHK_alumni]

Man does this sound spooky similar to the problems I've had with the 5.0 I put in my Ranger.

Nearly exactly the same compression numbers with the same engine work.

The one thing I could never get with compression numbers, at the 9:1 ratio of the Expo 5.0, I'm unsure mathematically compression numbers much better than 135 psi are possible.

In my case, was getting 13 mpg as a daily driver on the fresh 5.0, Torque Monster headers, OEM cam and mildly ported heads.

Fresh rings with the cylinder walls honed. New valve springs on lapped valves. Opened up the timing cover to assure myself of the cam timing and timed the cam position sensor while I had the engine at TDC.

I went so far as to replace the complete fuel injection system (except for the wiring harness), the cats and eventually the engine itself with a low miles used.

Still only getting low 15's as a daily driver. Way way off from what I'd expected.

Exhaust is OEM Expo 5.0 till the muffler where I'm using a 3" in/out Walker. Going to try a similar Dynomax 3" in/out turbo type but with a much shorter case and maybe a bit more rumble.

Even the used engine I installed is only offering up 130's for the compression.

Have gone so far as to mechanically add an additional 5° of timing advance, think I'm going to go even further. The plan is to add advance until I get a ping or two from the engine and then back off the timing till I clear the pinging with mid-grade fuel. That way, I can always pull the timing retard plug if I ever get a sudden bout of pre-ignition and can move up to premium fuel if needed.

 

[1997XLTRollover]

My truck has torque monster headers too, secondary cat delete and a magnaflow and still gets 10 mpg. This can't be right.

 

[Dono]

Its not right.
My 300,000km motor has 165-175 across all 8 cylinders.

Something is wrong

 

[1998Exp]

Yes, it's mathematically possible. Compression pressure being just the atmospheric pressure (about 14 psi) multiplied by the compression ratio applies if the temperature is held constant. However, as the mixture is compressed quickly, the temperature rises. If the valves and rings were to seal perfectly, and you waited long enough, the mixture temperature would finally drop and pressure reach about 130 psi. In fact, a better model for figuring the compression pressure in a cylinder is 'adiabatic' (no heat exchange). This provides for the pressure to rise much higher than being scaled by the compression ratio. Just my 2 cents. Sorry for the off-topic lecture!!!

...The one thing I could never get with compression numbers, at the 9:1 ratio of the Expo 5.0, I'm unsure mathematically compression numbers much better than 135 psi are possible.
...

 

[1997XLTRollover]

Its not right.
My 300,000km motor has 165-175 across all 8 cylinders.

Something is wrong

This is what I thought. My 99 runs strong, and only has a muffler delete and a Mac intake. My 96 has tm headers, a magnaflow and a fresh 306 vs my 99s 155k mile stock 302. Maybe I'll compression test my 99 too For the hell of it.

I'm pretty sure my 99 would take out my 96 in a heads up drag race.

 

[1997XLTRollover]

Is there any way for me to test to see if my cam timing is correct? If the sprocket is wrong how can I tell? I unfortunately already tossed my original timing set so have nothing to compare it to.

 

[Dono]

The only way I know of is to degree the cam and see where the cam events are. For a novice (as I am) I think it would be tedious work. I'm betting you could check it by just adding a degree wheel to the front of the harmonic balancer, and pull a valve cover. A spark plug piston stop would be needed to find true tdc also (no idea where you'd find that). Also, you need to know the factory cam timing events. Remember...I'm not the best guy to be giving you advice on this. You really need someone helping you that does this stuff all the time.