#1
|
|||
|
|||
1994 Altima: Are my valves bent?
Car is a 1994 Altima, 206K miles. The car will crank but not start.
Checked spark (good), fuel pump (felt pulsation when pinching the fuel hose), only code is 34, Knock sensor, which has been there forever. Timing chain not broken, as I see the camshaft rotating when cranking. No oil in the distributor. Compression numbers are bad: 100, 60, 100, 120 psi with engine cold. The battery is also weak, so perhaps that contributes to the bad numbers due to slow crank. I suspect the chain jumped a tooth (there was noise coming from the timing chain but did not sound too bad) and perhaps bent a few valves. Any ideas? Chris. |
#2
|
|||
|
|||
OK, I have more info on this one. After performing a leak down test, I believe I identified the problem to a very leaky head gasket. Every cylinder was leaking into the neighboring cylinder, I am amazed the car even ran. It does not appear that the chain jumped, so I believe the valves are not bent. I pulled the valve cover and the cam lobes look good, so maybe there is relatively little wear in the head.
I will attempt the repair myself. I will follow the procedures in the manual. I also plan to replace the chain and tensioners, I have the parts. Should I buy a whole gasket set? There are several aftermarket sets available. Are they any good or should I stick with OEM? Should I do the valve seals? What else should I be doing? Thanks for any advice you can provide. Chris. |
#3
|
|||
|
|||
One more question: are cylinder heads bolts in my '94 Altima reusable? The manual does not say to replace them, but the final step in the procedure makes me think they might be TTY (torque to yield) bolts.
Thanks, Chris. |
#4
|
||||
|
||||
If every cylinder is leaking into the next, I would have concerns of some serious warpage. I would send the head out to have it checked for flatness and cracks and then have a valve job performed if all checks out okay...or, consider a reman head, whichever option makes more practical sense. With 200,000+ miles on it, it doesn't make any sense to gamble. This way, you'll be sure all of the valves are good and you'll have the new valve seals installed.
The head bolts are TTY, and if you look up gaskets sets online, they will recommend replacing the head bolts. I believe there is a measurement spec for the bolts in the FSM. I would just get new ones. I would go aftermarket on a lot of the parts, but stick with name brands, like Mahle and Felpro. You'll want a head gasket set, a front crank seal, the seal between the block and the front cover oil pump port, and a lot of RTV sealant; I would go with Permatex Ultra Grey Rigid High-torque RTV sealant in a caulk tube, which is used with a caulk gun to lay down the sealant. Besides the timing set, oil, coolant and oil filter, I would also consider a new water pump, thermostat and drive belts while in there. I would do my shopping at Rockauto.com and if you need a 5% discount code, this is good until June 10th: 8132400157999262 |
#5
|
|||
|
|||
Thanks for your reply, very useful information.
I will certainly check the head when I remove it. The car never overheated, so I hope it will not be warped. Just to give you a bit more background: The car was noticeably low on power for a while now. It did not have trouble starting (but it would take a bit of gas when warm) and the idle was very smooth. I had not done a compression check (I should have) so I don't know if this was an indicator of the gasket going bad. Then one day it simply would not start. Only then I ran diagnostics, did compression and leak down tests, and got bad results all around. I was under the impression that gasket problems would give you plenty of warning, manifest as rough idle, trouble in starting, and rough running. I had none of that. The car was running smoothly, but low on power, which I had attributed to a bad knock sensor (ECU threw a code). Does all that sound normal? Chris. |
#6
|
||||
|
||||
No, not really. I've never seen a head gasket on one of these engines just fail all of the sudden without overheating and result in compression being passed into all of the adjacent cylinders. Sounds more like a jumped timing chain issue to me, but I'm only limited to what you're telling me and I'm not there actually do the diagnostics on the vehicle in person. When doing the cylinder leakdown test, were you able to figure out where the compressed air was going? Are you sure the loss was through the head gasket and not being passed by a valve into the intake manifold and into another cylinder(s)?
|
#7
|
|||
|
|||
I wish you lived near me, would be happy to pay you for a correct diagnosis!
The chain jumping a tooth was my initial suspicion. Now that you mention it, it is possible that air from the leak down test leaks from one cylinder to the others through the intake valves. There was never any noise coming out of the exhaust, and nothing from the dipstick. I removed the valve cover so I can see the cam lobes. I put the pistons at TDC using a stiff wire through the spark plug holes and turning the crankshaft until the wire would no longer rise. At that point the cam lobes would be horizontal, either facing opposite each other, or point at each other. Does that sound right? Can you help me with the correct procedure to determine if the chain jumped a tooth? Chris. |
#8
|
||||
|
||||
With the piston up, you will either be on the compression stroke or the exhaust stroke. On the compression stroke, both valves will be closed so the cam lobes will be in a position that they are not pressing down on the valves (obviously). On the exhaust stroke, the intake valves will be closed and the exhaust valves open, so the cam lobes should be pressing down on the exhaust valves on that cylinder.
Checking the timing involves putting the engine at TDC #1 and making sure the timing marks are in the correct position as per the diagram in the FSM. |
#9
|
|||
|
|||
I thought that when the piston is all the way up, both intake and exhaust valves will be closed, whether you are in the compression stroke or the exhaust stroke. Am I wrong? If I am right, then you can do the leak down test on two cylinders at a time, those whose pistons are all the way up.
Either way, when the pistons were all the way up (as indicated by the stiff wire I put through the spark plug holes) the cam lobes were horizontal, either pointing at each other or away from each other. So nothing was pushing down on the exhaust or intake valves. I will try again setting the #1 cylinder to TDC as shown on page EM 25 of the manual. That involves lining up the crankshaft with the 2nd mark on the pulley, the distributor firing at the #1 cylinder and the cam sprockets marks lining up with the chain marks. The chain marks have been a challenge because I do not have different color chain links, only some paint marks on the sprockets and the chain. Thanks again for spending time helping me. I realize that remote diagnosis can be very frustrating. Chris. |
#10
|
|||
|
|||
'I thought that when the piston is all the way up, both intake and exhaust valves will be closed, whether you are in the compression stroke or the exhaust stroke.'
No. Generally both valves are slightly open on TDC after the exhaust stroke, the engine is at overlap of that cylinder. FYI, it is generally easiest to bend valves at about 5-10 degrees on both sides of TDC, the intake at slightly after and the exhaust slightly before. The exhaust is almost closed at TDC and the intake has begun to open, the amount of overlap in degrees between both being slightly open is 'overlap'. FYI as well, any final bolt torque number that ends in degrees instead of a torque amount is a TTY bolt. Although at first some few were measured to possibly reuse they are commonly thought of as one time use bolts and if a head gasket has already failed there I would be getting new ones. Bolts that have stretched some already will not be at tight as ones that have never been stretched at all. Ford used to measure and reuse but encountered so many problems they stopped doing it. Like said a major head gasket fail like that normally has like an overheat tied to it. Last edited by Howard Bell; 05-07-2018 at 07:20 PM. |
#11
|
|||
|
|||
I am now thoroughly confused. If there is no point at which all valves are closed, how can we get a good reading during a leak down test (or compression test)?
Chris. |
#12
|
|||
|
|||
Both valves ARE closed on both sides of the TDC and at TDC itself at compression switching to power stroke.
|
#13
|
||||
|
||||
|
#14
|
|||
|
|||
Not very accurate there, you have a mini compression going on at the end of the exhaust stroke, the animation is closing the exhaust valve too early and the intake is too late in opening in the real world. Close though, it gets the general idea across. Valves would either open or close at the real TDC and BDC except that in the real world the fill and purge events have an element of time involved, the valves real world must open sooner and close later than thought, by the layman anyway.
|
Tags |
1994, altima, bent, valves |
Thread Tools | |
Display Modes | |
|
|