Hello all and thanks in advance for the help! Two issues I have with my son's 96 4 cylinder


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1. Engine is freshly rebuilt. Testing dry cylinder pressures, all 4 are about 165psi. When we attempt to start the motor it will start and then immediately die. When you try to restart I all of a sudden have no compression in any cylinder and the motor just free turns with the starter without the infamous "chug,chug,chug you normally hear when starting. If I let the motor sit for a little bit I suddenly start getting compression back one cylinder at a time. I can tell this by pulling the FPR and trying to start. I will her the single chug then free spin until it gets back around to the compression stroke for that cylinder. If I wait long enough all 4 cylinder gain compression again during start. It's almost like some sort of valve is opening up......wired....

2. Off the main harness is a square plug that is attached to the intake manifold. About 6 inches down the loom is a small plug that branches off that has a shielded wire on it so it looks like it may go to a sensor. For the life of me I cannot find out where it plugs into...

Any help is appreciated!!

 

Hmmmm. Sounds like oil pressure is building and the hydraulic lash adjusters are pumping up too far not allowing the valves to close all the way. When you are waiting the lash adjusters are bleeding down.

Was the wrong valve springs installed? Too weak allowing the lash adjusters to overcome them?

 

Thank you so much for the reply! When we rebuilt the head we put new lash adjusters in but used the old springs.......this may be my problem as the motor had 370,000 miles on it. I am wondering if the springs are weak now... Inspecting them I don't see a difference between the intake and exhaust springs and online it looks like all their seat and open pressures are the same.

 

If you didn’t have the same issue before the rebuild I wouldn’t think it would be the springs (at least not the main contributor)

can you give me more details on what all was done to the head during the rebuild. Was new seats installed or were the original ones just cut down and lapped?

The reason I ask is because I battled a temporary loss of compression issue for 2 years and finally figured it out to be recessed valve seats. On my cylinder 1 exhaust valve it was 0.030” below flush with the valve seat deck. 2 was 0.020” and 3 and 4 were flush. Reman head has them 0.050” over flush. So 1 was 0.080” recessed, 2 was 0.070”, and 3 and 4 were 0.050” recessed. Symptoms were stalling at red lights, stalling after hard acceleration, and rough/misfire at idle. If yours were cut to recessed them more than what I had, then I could easily see this being a problem.

 

Although I was having a similar issue with a newly rebuilt 2.5, start then die, wait and it will do the same song and dance........ I think the wire may go to a couple of things on the drivers side of the engine bay. At least these are the things I have on the 1998 that does not have A/C. Evap solenoid, mounted between the battery and power distribution box. The hose line that's attached to the plenum/intake bolt (the metal line between two flexible ones) is at the end of this. Also, hidden underneath the high pressure line of the power steering pump the sensor is sort of hard to find. It is located on the bottom of what looks like a metal to reinforced rubber coupling, right below the battery hold down bracket. It could also be for the camshaft sensor located on the side of the oil pump housing facing the timing pulley, hidden below the mounting bracket for the power steering pump and A/C compressor, but I am not sure your truck has one of those. There's a connection on the vacuum booster brake line housing as well as a connection to what may be the RABS, but I think the RABS is a four or five pin pigtail. As for the start and die issue, I'm still working on that one myself.........

 

I looked at the connection again and I believe Steban is correct. That is a two pin connector and the camshaft sensor is also 2 pin. If it's shielded then it most likely is as most variable reluctance sensors have shielded wires to protect the integrity of the AC signal they produce.

The best way to see the sensor is looking throw the driver's side wheel well. It's behind the oil pump sprocket as it picks up the "bend" in the backside flange of the sprocket.

 

Great Scott!!! I have a rebuilt 2.5 success story! The start then die issue is completely resolved...... the oil pump I purchased online for a better price actually had a flaw. I had the gasket on the oil filter pop on me and lost about 4 quarts of oil. Fine mess that made of the engine compartment, an adjacent trailer in the garage, as well as the floor. Luckily I had already put the hood back on so the ceiling didn't get sprayed as well. I at first thought that the gasket had not been seated properly in the filter so when it made contact with the block and I turned 3/4 more it just wasn't on correctly. Well a few weeks later it happened again in front of the house on the street. ZEP makes an awesome absorbent for these kinds of messes though. Got me thinking..... I know the second time I had put a new filter on it was on correctly and was actually on tighter than it should be. The new oil pump was constantly building pressure in the block, and the relieve valve/check valve that regulates how much the pump puts out must have been bad to begin with. Since the oil pressure sensor on the truck is just essentially an on or off switch, I decided to rig up a gauge to the port where it attaches to the head. I think the pressure built up enough to actually force oil past the new pistons and rings and into the combustion chamber. At that point the only way too get the truck running was to initiate a flooded mode with the pedal to the floor and wait until the chambers were clear enough to let spark and compression do their thing. Old pump in and problem solved. Although I'm a grandpa now myself, I feel like I need to add in some advice from my dad...... you get what you pay for.

 

That makes perfect sense. I am experiencing oil filter bloat and the engine dies until the pressure bleeds out. I did purchase the pump online so it was a cheaper unit. I guess the relief valve is in the pump so I’ll pull it back out and inspect it. Thanks for the advice!

 

I did find it, that’s exactly what it goes to.

 

Good deal, I hope you can get the same results with the oil pressure issue being addressed. My middle boy drove the truck I rebuilt 320 miles back up to college after the Thanksgiving break, and it ran beautifully. Still starts right up and seems to be a good rebuild so far. He'll be back down in a few more days and I'll change the oil and do a compression test on it.