Fresh rebuild will not start

  • 4.0L
  • 6 CYL
  • 4WD
  • 150,000 MILES
So, I am working on the vehicle listed above that I rebuilt the motor. Everything went together very nicely. Get everything back together and it will not start. First I made sure it has spark and fuel, and it does. Then I checked timing and it was 180 out, so corrected that issue. Still will not start, so I moved on to checking compression. All six cylinders were only at about 60 psi, so I went on to doing a cylinder leak down test and that showed no leaks at all. So that tells me two things; motor is good and timing chain is on correctly. For about a week I have been cranking it over about five times a day and now the compression is roughly about 85 psi and it sounds like it wants to fire on a couple cylinders. What am I missing?
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have the same problem?
Wednesday, August 15th, 2018 AT 9:56 AM

1 Reply

HI Justin and thanks for using 2CarPros. Com.

The engine does not have enough compression to run. The manufacturer indicates it should be between 120 and 150. From experience, if it is below 100 it will not run or if it does, it will not be good. I attached pictures indicating general specifications for this engine.

I am not sure why the engine was rebuilt. If it was due to low compression, take a look at these directions for fitting rings on the pistons. It shows how and what the tolerances are for each step. The pictures starting at number 4 correlate with these directions.


1. Carefully clean the carbon from all ring grooves. Oil drain openings in the oil ring groove and pin boss must be clear. Do not remove metal from the grooves or lands. This will change-ring-to-groove clearances and will damage the ring-to-land seating.
2. Be sure the piston ring grooves are free of nicks and burrs.

Fig.7 Piston Dimensions

Fig.8 Ring Side Clearance Measurement
3. Measure the ring side clearance with a feeler gauge fitted snugly between the ring land and ring. Rotate the ring in the groove. It must move freely around circumference of the groove.

Groove Height

A 1.530 - 1.555 mm (0.0602 - 0.0612 inch)
B 4.035 - 4.060 mm (0.1589 - 0.1598 inch)

Top Compression Ring. 0.042 to 0.084 mm (0.0017 to 0.0033 inch)
Second Compression Ring. 0.042 to 0.084 mm (0.0017 to 0.0033 inch)
Oil Control Ring. 0.06 to 0.21 mm (0.0024 to 0.0083 inch)

Fig.9 Gap Measurement
4. Place ring in the cylinder bore and push down with inverted piston to position near lower end of the ring travel. Measure ring gap with a feeler gauge fitting snugly between ring ends.

Top Compression Ring. 0.229 to 0.610 mm (0.0090 to 0.0240 inch)
Second Compression Ring. 0.483 to 0.965 mm (0.0190 to 0.0380 inch)
Oil Control Ring. 0.254 to 1.500 mm (0.010 to 0.060 inch)

5. The oil control rings are symmetrical, and can be installed with either side up. It is not necessary to use a tool to install the upper and lower rails. Insert oil rail spacer first, then side rails.

6. The two compression rings are different and cannot be interchanged. The top compression ring can be identified by the shiny coating on the outer sealing surface and can be installed with either side up.
7. The second compression ring has a slight chamfer on the bottom of the inside edge and a dot on the top for correct installation.
8. Using a ring installer, install the second compression ring with the dot facing up.
9. Using a ring installer, install the top compression ring.
Position the gaps on the piston as shown.
- Oil spacer - Gap on center line of piston skirt.
- Oil rails - gap 180 apart on centerline of piston pin bore.
- No. 2 Compression ring - Gap 180 from top oil rail gap.
- No. 1 Compression ring - Gap 180 from No. 2 compression ring gap.

Now, to determine if the compression loss is due to cylinder wear or ring issues, or if it is related to leaking valves, you need to do a wet compression test. This is done by injecting an small amount of oil, usually about a tablespoon, into the cylinder and then checking compression. If there is an increase in compression, the problem is with either the rings or the cylinders have excessive wear allowing air to bypass. If there is no change, either there is a problem with the head gasket or the valves in the head.

Do this and let me know what you find.

Take care,
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Thursday, August 16th, 2018 AT 7:26 PM

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