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So I bought a junkyard LQ4 really cheap to eventually put in my 83 BMW 533i that currently runs a GT40 5.0L Ford. I was able to cold compression test it today using the starter and some jumper cables. Disappointing numbers followed: 4 @ 130 psi, 3 @ 120 and 1 @ 110. After that i pumped up each cylinder (one at a time) to 15 psi and brought them to TDC to listen for leak down. Every cylinder is leaking at the valve seats and you can hear it pretty easily. Not sure if they are all burnt of just covered in crap / rust from sitting in the yard. Anyhow, since the LQ4 will eventually end up in my BMW and I still need to build the standalone and find a ECM i though id like to do that and test before tearing into the motor.
So long story short, here's the question. Will the LQ4 run well enough to at least see if the ECM and DIY harness is working with compression numbers that low?
I was going to build a run-in stand anyhow so its not like i am putting an engine I know is tired into the car. I will ultimately rebuild it but i think i'd like to build the harness and have the ability to run the engine first.
Yes it will run. You won’t start to have problems until less than 90 psi. Try putting about a tbsp. of motor oil in each cylinder and retest. I bet the cylinders are dry and a little oil will help.
At this point with it sitting on a stand I'd spend 125 bucks to get new TTY bolts and head gaskets.
Yank the heads to check/clean/inspect
You already know it sat for a long time so pulling the valves and springs apart can only help. Clean everything, lap the valves back into the seats and perhaps toss some new valve seals in there as those can dry rot and cause smoking issues from sitting in a scrap yard for lengthy periods.
You wont get 200 psi from it but it will definitely run/tune better if it ends up in the 150-160 range.
Yes it will run. You won’t start to have problems until less than 90 psi. Try putting about a tbsp. of motor oil in each cylinder and retest. I bet the cylinders are dry and a little oil will help.
I'll give that a try for sure. I had the crankcase full of oil and had a mechanical oil pressure gauge connected. Spinning it with no plugs in got 25 psi on the gauge but it took a while before all the rockers were oiling. I dont think the starter can spin it fast enough to generate more than 25 psi. Oil was ice cold so it was thick and hard to pump.
Originally Posted by 96capricemgr
Was the tb held open?
Yes, tried it both ways with only minor pressure differences
Originally Posted by A.R. Shale Targa
At this point with it sitting on a stand I'd spend 125 bucks to get new TTY bolts and head gaskets.
Yank the heads to check/clean/inspect
You already know it sat for a long time so pulling the valves and springs apart can only help. Clean everything, lap the valves back into the seats and perhaps toss some new valve seals in there as those can dry rot and cause smoking issues from sitting in a scrap yard for lengthy periods.
You wont get 200 psi from it but it will definitely run/tune better if it ends up in the 150-160 range.
I think that may well be what I do. I know it was leaking at the valve seats for sure so lapping those valves can only help and the cost to do it very little. This way I can see if the cylinders have a ridge and will need to be bored when I do finally rebuild it. I know someone has been in this thing before; there are signs that at minimum the heads have been off:
Two hardware store grade bolts in the exhaust manifolds.
Two oddball bolt/nuts holding on the manifold downpipes (studs sheared and drilled out?)
Marks on the heads on the intake side from a Roloc or other stripping disc.
Blue headgaskets (fel-pro?)
Engine came from a 2006 GMC Savana 2500 FedEx van. Cluster was gone so god only knows how many miles it has on it or how FedEx's mechanic of choice took care of it. Block VIN matches the Van so its original and been torn into before. The undersides of the valve covers were significantly dirtier than the heads / valve train so I'm thinking the heads have been gone through at minimum once before.
I pulled the engine apart and cleaned the piston tops. Cylinders look ok, no ridge at all. No scoring or pitting that I can see but no crosshatch remaining either. Someone has definitely had the heads off before, there are roloc marks all over the heads on the gasket surface. The marks are significant enough that I'd be concerned about the head gasket sealing right. Doesn't look like the gaskets were blown though.... Pistons on each bank are marked differently:
2-8 are marked: 7233 P180 5205 190
1-7 are marked: 7233 R179 5206 220
Anyone know what all that means or why they are different?
Factory pistons have offset wrist pins which reduces piston rock (slap) on directional change. It also lessens ring flutter which helps seal combustion pressure better promoting less emissions. This is why the dot faces the front of the engine.
Get a spring compressor and pop the heads apart. Have them surfaced as the roloc crap can damage the gasket sealing surfaces. If I were you I'd have them surfaced .030" to get 10:1 compression...and if you're up for a quick cheap improvement, pickup some blue or yellow GM valve springs from a vendor like Summit and I have a LS2 cam and some real low mileage valve seals that have been soaking in motor oil. IIRC the springs are only 60 bucks and I'll ship you this cam and seals for cheap.
specs are 204/210 @ .050" and .525 lift. With headers and a tune 425 fly wheel horse would be a great starting point till you build it up at a later date.
I have a LS2 cam and some real low mileage valve seals that have been soaking in motor oil. IIRC the springs are only 60 bucks and I'll ship you this cam and seals for cheap.
Wouldn't the LS2 cam require swapping the front cover and cam gear as well since the LS2 doesn't have the reluctor cut into the cam?
04-25-2015, 07:35 PM
Will it run? 
