Engine vibration ONLY around 1800 to 2400 rpm under load

[AC439]

I have been trying to figure this one out for a while.... 86 LXi automatic.

The symptom is when the car is in 4th gear, the engine will vibrate between 1800 to 2400 rpm. Any rpm below or above seems fine. I know this is under load condition (4th gear) but strange is only at a certain rpm range. I read some posts (here or other forums) people are saying motor mounts. My mounts are relatively low miles and the engine really not giving much vibration at idle so I don't think my mounts are bad.

It starts happening after I tightened the head bolts few months ago. I have since bought a torque wrench and re-torque all the bolts few weeks ago to spec but no improvement. I also adjust valve lash, new VC gasket. So far, I have swapped out my set of NGK wires (over 10 years old). Oil change to rule it out.

I also cleaned the PCV valve well, dissolved all the gums inside using brake cleaner. Rattle test is good (I know, it doesn't always mean the valve is good). The groumet is original. Overall the fitting is ok but there seems to have some play around the PCV body. I know this can be a problem due to vacuum leak and I know the PCV should not have air leaking around it. Since the groumet is old part and unable to find replacement, I thought about relocating the PCV to connect to the valve cover and plug the groumet. I don't think I have read anyone done this here. Or, I may want to wrap the PCV with electrical tape and see if this will make a difference just to prove my theory.

Before new spark plug wire and PCV cleaning, it was pretty much happening 100% of time. Afterwards, it is intermittent, maybe 60% of time.

I have a new air filter to put in (but I doubt this is the issue because existing filter is not too dirty) and also thinking about swapping a set of plugs (NGK G-power, currently Autolite double plat).

Not sure if there are other things I should check............. Any thoughts ?

[InAccordance]

How many miles on the transmission?
Mine shudders at low rpm under heavy load as well but I know mine is getting ready to go cause it is slipping from 2 to 3rd gear.

[AC439]

My tranny is original 321k miles on it and I only use Honda ATF change every 10-15k miles faithfully. It does not slip at all.

I did my electrical tape around PCV just an hour ago, test drove it and still occasionally shuttering at those RPM range.

Next try is to swap out a set of plugs............

[carotman]

It's at those RPM at any speed/gear?

It does it in 3th gear too?

[AC439]

Most noticeable at 4th gear (automatic). I changed out to a set of NGK G-power today, still got vibration but a little less. So this is still not the answer....

[conozo]

I would say check the egr valve. Try disconnecting the vacuum hose to it and plug the hose to see how it drives.

[AC439]

I was also just thinking about the same after some more digging on the internet. I've never seen code 12 (EGR fail) but I have never taken the EGR out either. It is an original part. I want to at least take it out to clean it. The two nuts are not easily accessible, the top of the whole assembly is blocking the nuts. I read somewhere I need a u-joint adapter to get it out. I would imagine this is going to be hard and probably a lot of !@#% in process of removing it. I prob will need to soak it with PB blaster overnight.....

[AC439]

Installed the new air filter. Old one is dirty but not enough to make me think this is the cause of the problem. Car runs better afterwards but still have vibration but seems less. So far, everything I did seem to help a little (older worn out parts adding up).

I have to mention I am still having the rear motor mount on the front (stated in my older thread). But it has only been installed for less than 3k miles ago. Well, this could very well be it if after cleaning the EGR as planned for next step....

(I cannot find OEM mount, went to a dealer part dept and they said part discontinued). Next is to use Energy suspension or if I can get the cast iron front bracket, I can then use the carb mount instead....

[InAccordance]

You replaced an engine mount?

[gcovarrubias412]

Installed the new air filter. Old one is dirty but not enough to make me think this is the cause of the problem. Car runs better afterwards but still have vibration but seems less. So far, everything I did seem to help a little (older worn out parts adding up).

I have to mention I am still having the rear motor mount on the front (stated in my older thread). But it has only been installed for less than 3k miles ago. Well, this could very well be it if after cleaning the EGR as planned for next step....

(I cannot find OEM mount, went to a dealer part dept and they said part discontinued). Next is to use Energy suspension or if I can get the cast iron front bracket, I can then use the carb mount instead....

Wait, you’re using a rear motor mount for the front? Thats most likely whats gonna be causing vibrations.

Also, I think those rpms are where the car makes the most torque, or atleast close to peak torque

EDIT

Nevermind, you said it happens in 4th gear. That has the least amount of torque lol. Maybe one of your mounts is loose? Pretty sure it’s gonna be that mis-matched mount

[Dr_Snooz]

If this issue started after the mount install, then I'd be looking real hard at that. There used to be a thread around here about using a generic mount. I'm not sure if that would work here, but might be worth reading just for general info. You should also check tire wear and balance, suspension condition, etc. If all that checks out and it's only at specific speeds, in specific gears, at specific RPMs, then I'd be thinking transmission. Are you having any other transmission issues?

[Dr_Snooz]

Here it is.

https://www.3geez.com/forum/restoration-custom-fabrication/72765-alternative-poly-engine-mounts.html?highlight=transmission+mount

[AC439]

Thanks for all replies. I am well aware of that thread about generic mount and I am about to order the energy suspension tranny mount and make a adapter for it.

The vibration did not start when putting that rear mount at the front. But that doesn't mean it will not break (someone mentioned this possibility in another thread) after installation.

The vibration does happens in other gears but most noticeable 4th gear around 2k rpm. At 4th gear, the engine works hardest so I think the vibration is more pronounced. Other lower gears the engine still vibrates at 2k rpm but a lot less noticeable.

I will post results once I can get the energy suspension mount fit as front mount.

[gcovarrubias412]

Thanks for all replies. I am well aware of that thread about generic mount and I am about to order the energy suspension tranny mount and make a adapter for it.

The vibration did not start when putting that rear mount at the front. But that doesn't mean it will not break (someone mentioned this possibility in another thread) after installation.

The vibration does happens in other gears but most noticeable 4th gear around 2k rpm. At 4th gear, the engine works hardest so I think the vibration is more pronounced. Other lower gears the engine still vibrates at 2k rpm but a lot less noticeable.

I will post results once I can get the energy suspension mount fit as front mount.

So when did the vibrations start?

[AC439]

So when did the vibrations start?

Funny you asked this.... As far as I remember, it started right after I remove the valve cover and tightened the head bolts. I found the middle 2-3 bolts seemed to be able to be tightened just a little. I have since used a torque wrench and re-torqued all the head bolts to spec again (loosen them up a little, then torque according to spec/sequence). But the vibrations still there at the specific rpm and load.

[gcovarrubias412]

Funny you asked this.... As far as I remember, it started right after I remove the valve cover and tightened the head bolts. I found the middle 2-3 bolts seemed to be able to be tightened just a little. I have since used a torque wrench and re-torqued all the head bolts to spec again (loosen them up a little, then torque according to spec/sequence). But the vibrations still there at the specific rpm and load.

Hmm. Well, i know that when you remove the head off an engine, you need to remove the head bolts in a particular order so that the head doesn’t warp. That might be it? Remove the head in the certain order the bolts need to be removed, then reinstall it in the correct installation order.

[AC439]

Hmm. Well, i know that when you remove the head off an engine, you need to remove the head bolts in a particular order so that the head doesn’t warp. That might be it? Remove the head in the certain order the bolts need to be removed, then reinstall it in the correct installation order.

I did torque them in the order and spec as stated in the FSM. I hope this is not due to the head wrap.