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Bad engine vibration at low RPMs only 89 4.3

8K views 16 replies 8 participants last post by  Evilcowboy420 
#1 ·
I've posted similar problems about this before but figured I'd start a new part some I narrowed down some things.

What would cause an engine to shudder/vibrate bad at low RPMs? It's a TBI 4.3 at 68k miles. I don't have a tach so hard to say, but it's pretty much as soon as I give it a little gas it vibrates bad and then when I bump up the rpms a little more it runs smooth and perfect. Idles a little rough sometimes too. All this happens whIle in park so I know it's the engine.

I've been told I just need to rebuild my engine cuz it's old and probably has bad rings or torque converter. But is there anything I can test before jumping into a rebuild?
 
#3 ·
How was it before it started to this, smooth, or did you notice/feel something different before it got bad. Did it start doing it while driving, or just started doing it one day when you started it.


Someone told you because it vibrates, it needed to be rebuilt because it was just to old, or maybe has bad rings, or a bad torque convert, doesn't have a clue as to what's wrong with it!


Pat
 
#4 ·
Well, hard to say since it's not my driver. I just got my driveshaft balanced a little bit ago and that fixed major vibrations at high rpms. I remember feeling that vibration at low rpms a little bit before I had the driveshaft fixed, so now this really sticks out. I'd say it happened fairly quickly though considering last fall it wasn't like this and the low miles I've put on since.
 
#5 ·
Check your motor mounts - put it in drive while holding the brakes and give it a little throttle and do the same in reverse watch the engine and look for it too rock - even with good mounts the engine will move a little but with a bad mount/s it will be very noticeable -
 
#6 ·
If you fouled a spark plug (4.3s tend to use oil from bad valve seals) it will cause a vibration at low engine speed - running on 5 cylinders. Once you pick up engine speed, it usually becomes smoother even though it's one cylinder down.


Wait - didn't you replace your valve seals?


Check your plugs just for the hell of it.
 
#9 ·
I did put new valve seals in recently. Also, harmonic balancer is all good as far as I can tell. But now that I think about it.... plugs can probably fowl up pretty quick? I've been doing lots of testing and had several issues I've fixed over the last few months. I put new plugs in earlier, but with all the problems, including gas getting in the oil at one point (I flooded the engine during testing). I of course changed the oil, but didn't think about the plugs... aside from me being pretty ignorant when it comes to visually inspecting plugs.

Ugh, I hate when I sometimes overlook the obvious, then it's embarrassing when I go on here to ask, sorry about that.

I was out at a Bachelorette party last night, so feeling a little under the weather today. But I think if I get out and work on my truck it just might be a good cure haha. I'll check the plugs and do a compression test if I can rent that. I ran a test about a year ago with i though good results, but can't remember exactly, so I'll do another.
 
#10 ·
A static compression tester is like $20 and should be a part of any persons tool box. A leak down test is better and can be made from an old spark plug: knock the porcelain out, weld on a compressed air fitting, and hook it up to the hose off your air compressor. If one cylinder is a little lighter on compression, this can be somewhat compensated for with a slightly larger gap on that cylinder's spark plug in order to ignite it.

Pull plugs, clean with a wire brush and some gasoline. Make sure to check their gaps.
 
#11 ·
So I'm back with my compression test results, got 125 on all and lowest was 115 on number 6. I thought that was bad at first, but read that it's normal on mine.... I inspected all the plugs and all good, but 1 looked bad like carbon and almost looked like a couple tiny bits of the old valve seals were stuck in it (if that's possible) so replaced that. I didn't do a leak down test though. Good idea about making my own, but I don't weld or know anyone who does.
 
#12 ·
Is it possible one injector is dumping gas into that cylinder with the fouled plug? I noticed the oil smells a little gasy and I just changed it a couple weeks ago. The injectors are not leaking and they appear to be spraying the same but when I rev it up slightly I sometimes hear a loud SHH sound, is that normal? And could that be the cause? No check engine light on btw.
 
#13 ·
Personally I would think if one of the injectors is dumping gas it would be more than just one cylinder affected. Sounds a little like one of the valves may be stuck open. Check to make sure raw gas isn't getting thrown out the exhaust.

As far as the SHH sound first thing that springs to mind would be something like a vacuum leak somewhere.

I would check over the throttle body as well lots of little electronics that is not working right would cause the air/fuel mix to be off. Oh and make sure your tbi's fuel regulator hasn't gotten a ruptured diaphragm. That can cause it to dump gas into the intake.
 
#14 ·
That could be it. I just did a rebuild on the TBI but double checking that diaphragm and spring would be a good idea just to make sure nothing got kinked.

I also need a new distributor (one of the screw holes is stripping) I wouldn't doubt that could be causing some problems too. So I'll get a new one there.

I've already done new valve seals myself, so my skill level is going up and i figure while I'm in there why not remove the intake manifold and the heads to inspect and clean? I know removing the manifold is easy while the engine is still in, but would it be too tight of a work space to remove the heads?

I don't mind doing all the work even if it's not necessary cuz I figure I'll at least find out if there's any problems on the upper part. And if need be I'll go ahead with a total rebuild from there. :)
 
#17 ·
The only bad thing about pulling heads off is I can guarantee you will find a couple of bad exhaust valves. Once you see them all pitted you will then want to do a valve job. Once I had a engine that I pulled one head off because the piston head broke. I then decided even though the rear head held compression on all cylinders I should at least replace its gasket. Upon removing it even though it ran fine on that head I found a small crack between the intake and exhaust valve on that head.

The moral of that story is more to prepare you for the possibility of needing to replace a head or finding something you really wish you hadn't. Don't pull it apart unless you are absolutely certain the problem isn't something simple.

Also if you plan to remove the motor I always consider camming the engine and think if it is feasible and if it will be worth the money or if the gains is simply minute and not worth it.
 
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