Missing when in gear, though not in park

jtettenb · 10-16-2009

I have a 289 with barely three thousand miles on it. I've never been the best at tuning and so it has taken me that long to get it really close. The car runs great at speed, and when parked and idling it seems to run smoothly as well. ( it is a little shaky but not too bad) It gets really bad when Im in drive stopped at a light or pulling slowly through a parking lot. It sounds like one of my cylinders is missing. of course when I put it in park it goes away making it so I cant tell which one it is. I just quickly checked the plug wires and they seem fine. I pulled the wires from the cap one by one while running and every one of them dropped the rpm. ( suggesting to me that all the plugs are firing) Is it possible thats wrong and one or more of my plugs is already bad? (it is a newer engine) or is there something else it could be. I just cant seem to find a reason why it does this in drive but not in park. Thanks for any ideas you can come up with

429ragtop · 10-16-2009

I would do a full compression and leakdown test, sounds like the start of a blown head gasket. Other things I would check are vaccum leaks (power brake booster if you have one, tranny modulator, pcv valve, etc), Ignition system, and lastly the torque converter. But I would check compression first.

429ragtop · 10-16-2009

By the way........ GO DEVILS!

jtettenb · 10-17-2009

Its good to hear from a sundevils fan! you were right on the money; I did a compression test and found my number 5 cylinder was down to 115 psi. doing the test wet brought it right back up to normal. I cant say how disappointed I am to find that its the piston ring. This is a newly rebuilt motor with less than a few thousand miles on it. Do you have any idea how this might have happened? ( this way I don't do it again) Also, should I replace all the rings? I'm guessing I should but with so few miles on it, it just seems a waste. Thanks for any advice you can give.

429ragtop · 10-18-2009

What was the compression on all the other cylinders? Why do you suspect the ring?

Print Dad · 10-18-2009

Quote:

Originally Posted by 429ragtop

What was the compression on all the other cylinders? Why do you suspect the ring?

Hello 429ragtop,
I just wanted to say you did a great job on this thread and keep on him to figure out what is going on.......... Print Dad

jtettenb · 10-18-2009

155 145
155 115
155 150
155 150

These are the numbers I came up with when I ran the compression check. I could be off by five or six in either direction just because the numbers on the dial are small and so I attributed the slightly lower numbers on the right side to that. When I added a little oil to the cylinder in question (115) the compression bumped back up to around 150. I thought this could only mean a bad ring. If you tell me i'm wrong I just might jump out of my seat with excitement.

429ragtop · 10-18-2009

The next step is to do a leak down test on cylinders 5 through 8 to confirm and isolate the problem. Make sure you use a proper leak down tester, and do the test correctly.

jtettenb · 10-19-2009

Ok. It may take me a day or two to buy a leak down tester and learn how to use it properly. But ill get started first thing tomorrow. Thanks for all the advice. Ill let know how it goes. Hopefully its something easier than a ring.

429ragtop · 10-19-2009

harbor frieght tools has a good one for cheap.

The Greek · 10-20-2009

While it's a good thing to do a leak down test, it's not completely necessary at this point. A leak down can pin point leakage in the valves, head gaskets and also the rings. If you have increased compression after oil is introduced into the cylinder, this indicates rings that aren't seating properly. The oil would not change the compression if it were valves leaking or a head gasket.

You can have rings leaking from having a surface in the bore that lacks the proper crosshatch pattern and depth when the cylinders were honed. The rings won't seat properly at break-in if this is the case. Rings installed upside down, too much bore diameter for the piston and/or rings and an out of round cylinder can contribute to this condition. If a ring were broken, I doubt that adding oil would have much of an effect on compression.

With so few miles on the engine, I wouldn't rework all of the cylinders. Just concentrate on the cylinder that's leaking. If you had someone do the engine's build, don't let them get away with this, because this is going to involve some work to fix. The compression increasing with the addition of oil definitely points to rings.

429ragtop · 10-20-2009

[quote=The Greek;1757881]While it's a good thing to do a leak down test.

jtettenb · 10-20-2009

So I went to harbor freight and picked up a tester. I spent half my day researching how to use it properly and the other half of the day trying to figure out why I couldn't get it to work right. After searching the forums and doing some easy tests on the piece myself I concluded that the tester I got is garbage and ill need to exchange it for a new one. (After searching online it seems this is not unusual for this brand of tester) So ill need another day to rerun the test just to insure without a doubt its the ring. Now my question continues, if it is the ring, can I just replace the ring or will I need to re-hone the cylinder? I'm guessing I know the answer to that one though

Ill rerun the test and let you guys know how it goes. I have to thank you again for all the help you've been with this matter, I don't know what I would do without the advice I've received from this site. Though the cars not running properly right now its a long way from the junker I bought a few years back. Thanks to you guys

The Greek · 10-21-2009

What does 1757881 indicate?

jtettenb · 10-24-2009

I have no idea what does that indicate? I don't follow