Engine vibration at certain RPM

I agree. Check the balancer.

I have a 93 GT vert, having this same problem; getting a strong vibration/shake through the steering column between 1300-1400 RPM. Occurs whether in gear, or not moving in neutral - clutch in, clutch out, doesn't matter. My car is basically stock, no crazy mods. Took it to a mechanic, the balancer was wobbling all over the place, so it was replaced, but did not fix the vibration. Then I noticed the radiator fan was cracked in several places. Replaced it and the thermal clutch just to be sure - engine runs much smoother and has more power now, but vibration at those RPM's is still present.

After taking it for a longer drive today and really focusing on how everything sounded and felt, determined the following: vibration is worse when the car is cold (live in So Cal, so not COLD, just when it hasn't been running awhile). By the time I got home and tried to make it shake again in neutral, it was nowhere near as strong, but still there. Also, the vibration is definitely traveling through the steering column - the steering wheel is shaking up and down, with some vibration getting transferred to the dash as well.. The engine itself and nothing I can see on it, is vibrating at all - I looked carefully while manually raising the throttle in the engine bay.

Lastly, I also ran KOEO/KOER and cylinder computer tests. All cylinders came back good, only EEC codes were 44/94, which I've gotten ever since I first pulled codes a couple years back. At some point when I have nothing else to do, I'll replace the crossover tube to see if those go away.

I am thinking the steering vibration has to be coming from the PS pump and/or PS rack. Both are original, and even though the car only has 77K miles on it, it's almost 30 years old now. Another factor is that there is a leak in the front of the engine compartment, approximately where the pump/steering rack are located. Lastly, the ride on the front end of the car has always been extremely harsh. Any little bump in the road sounds/feels like I'm driving over a steel beam. PO had lowered the front end a couple inches, I had the springs and struts replaced to OEM specs but no change in ride quality. The rear end of the car rides/feels as it should. There are subframe connectors and cross members from the PO. They are welded.

For background, I've had the car just over two years, I'm the third owner. This is my second Fox vert, having had an 86 5.0 LX back in the mid 90s which was 100% stock/original, so I'm familiar with how it should feel and act on the road.

I've searched and searched, and read many posts here. Unless there's something else I've missed, I'm ready to upgrade to a new SVE PS rack and also replace the PS pump with a remanufactured Motorcraft. Will have all hoses etc. replaced too just to make sure everything steering-related is new again.

Would appreciate any thoughts/feedback. Thanks!

Check your motor mounts.

Thanks Karthief and 93SSP. A little more info and answers to your questions, bear with me for the long explanation. Trying to be as thorough as possible:

Yes, can feel the vibration at idle and while revving engine slowly. That's how I narrowed it down to the 1300-1400 RPM range. After listening/feeling closely today, I think the vibration is there even at lower RPM's, from idle all the way up to 1400 RPM. It's barely distinguishable from regular engine vibration, but it is there. It becomes severe between 1300-1400, and then goes away completely once the engine goes above 1500. It mainly comes through the steering column and wheel, you can actually see the wheel vibrating when it happens. But I can also feel it coming through the body a little, if I hold my hand on the door panel when it's happening. Again today, I revved the engine slowly with the throttle while looking and feeling around in the compartment, and couldn't see or feel a vibration in there aside from typical engine feel.

It's definitely more noticeable when I first start the car in the morning. This morning, I went for a drive down the street to the post office, then looped back a mile-plus on the freeway. So the engine was warm when I got home, and the vibration had gone away a bit but was still there.

Engine runs smooth, no misses or rough idling etc.

Regarding motor mounts, they were replaced shortly after I got the car a couple years ago. I then went to a different mechanic to do the harmonic balancer earlier this year. When that didn't make the vibration go away, he suggested the motor mounts be changed, even though he could see nothing wrong with them. He did say they were metal mounts, not poly or rubber. According to him, the vibration wasn't there when the engine was lifted and running off the frame, which is why he thought they might be the culprit. He was going to order new mounts to swap out and see if they made a difference but we didn't get around to it. Would mounts be causing this issue only at RPM's below 1500? I would think bad mounts would have the motor shaking even worse at higher speeds, not allow the vibration to go away. Or is it because the engine is turning so fast that the metal mounts would be fine at higher speeds but lack any sort of dampening at lower speeds when there's more time, however small, between revolutions?

In addition to the leak in the PS pump/rack area, I should've also mentioned that when I opened the pump reservoir, the fluid looks like chocolate milk and smells off. Not burnt, just off. I assume that definitely indicates a worn pump or bad seals somewhere. When I had the oil changed a couple months ago, they said the PS fluid was low and topped it off for me. They also said there was a transmission leak, but I've seen no sign of one before or since, so they probably confused ATF from the PS with the transmission.

Also, I'm not experiencing any other steering/control issues. No excessive play in the wheel at any speed, no pulling, it'll stay true if I take my hands off the wheel. There was a brief moment last year I think where it was almost like the PS lost power while I was slowing down or braking, and it did pull hard to the right, but it came right back after I gave it some gas - never happened again after that.

Lastly, as I mentioned previously, the PO had the front end lowered a couple inches. There were also no rack limiters installed. He had to stop me from rubbing the tires when I was cranking the steering and driving it the first time. So I had a rack limiter put in on each side, same time the motor mounts were done a couple years ago. I've also read that lowering the front end without installing a bump steer kit can damage the tie rods and/or rack, so I'm guessing there's at least something wrong there, but no mechanic said anything about those components, unless they failed sometime after that.

So question is, I guess, could the vibration actually just be bad motor mounts (even though they're newer), and the steering leak/pump/rack issues if any, are just a coincidence? Are there specific mounts you'd recommend be swapped out the next time I take the car in? And I will probably have them closely examine the PS pump and rack, or just replace both at the same time.

Only other variable is the vibration going down once the engine is warm. Will a hot engine vibrate less than a cold one since everything has been moving around awhile? And if it's at all related to the PS pump/rack, then I can see how if they're failing, they work better once the fluid has a chance to circulate.

Thanks for any suggestions/feedback!

You say it vibrates the column and wheel? For s and giggles, take the serpentine belt off. Maybe it's the p/ s pump?oh and if so flush that beach , before you replace it.

Had a similar issue with vibrations at certain rpm's on an 82 Capri RS 5.0 and it was the harmonic balancer. Then it got a whole lot worst eventually. The rubber got separated from the metal balancer. Replaced the balancer and was smooth, but not like butter until I found a vacuum leak, then it was smooth as butter.

Thanks all for the ideas. Removed the serpentine belt and started it up this morning. Vibration was still there at the same RPM's as before. Maybe not as strong, but definitely present. Could be because it didn't get that cold here overnight. So we can rule out the PS pump as the cause, though there still may be something wrong with it due to the fluid and leak.

Will look into the firing order next just in case. It had a tune up right after I bought it - new plugs, wires, and I think maybe a new cap too. SO unless that guy hooked things up wrong, that shouldn't be an issue.

I looked down at the motor mounts today also. As far as I can tell, they look like they have some kind of cushion in them - couldn't tell if it was rubber or poly.

I've looked around at the vacuum lines before and haven't found any cracks or loose connections. Had to replace a couple because I broke them myself. They got replaced with brand new ones, same diameter, etc.

This vibration issue predates all of the above - it's been present ever since I got the car over two years ago. It's really the only nagging problem it's ever had, and multiple sensors/parts have been replaced out of an abundance of caution, and figuring they'd need it sooner or later. All of the vacuum solenoids, the EGR, the PCV, IAT, BAP, and O2 sensors have been replaced. And just did the fan and clutch. I've also cleaned out the MAF sensor, throttle body, and have a K&N OEM style air filter in the stock air intake that I put in last year. The only EEC codes ever thrown out are the 44/94 for the crossover tube, but I've put off fixing that since it's not supposed to be a big deal.

I was getting really poor city mpg, and subpar freeway mpg until the O2 sensors got replaced this summer. PO had put in Bosch sensors, and I replaced them with new Motorcraft ones. Since then it's averaged a solid 16-18 mpg with average driving around, and I got 25 mpg when I took a 200 mile round trip on the freeway.

Engine was smoother after the new O2's, and then once I realized the fan was cracked and replaced that, it got really smooth. Except for that RPM vibration still coming through the steering column.

Question - since I know there's an issue with the crossover/smog tube, and that's the only error code I've ever gotten on the engine, plus the fact the vibration is worst when the engine is truly cold, could a blockage there or at the back of the intake manifold where it connects, cause that vibration when the air can't get through there the way it should? Since the solenoid valves close and bypass air straight to the exhaust once the engine is warm, and that's also when the vibration is nearly gone - could that be the answer?

Quick update: firing order is correct, checked out the position of the distributor cap and traced each wire.

Also took a closer look at the motor mounts, prodded around with a screwdriver. They have a cushion in them, the flat blade screwdriver could push into the material slightly, and look to be in good shape.

Well you have replaced a lot of stuff,so why not go for a new balancer? Heck
they aren't expensive and if it's as old as the car chances are it's took a crap.

Balancer was bad, and was replaced earlier this year. I mentioned it in my first post. I did look at it again just in case, but it still looks new and not vibrating or anything.

Idle was a tiny bit rough this afternoon when I was checking things out again. I mean just barely rough. RPM's were fluctuating maybe 50-100 or so at idle. Almost sounded like a carbed engine. So maybe a vacuum test is the way to go.

Okay @General karthief @Mstng93SSP, have a follow up for you if you have any more suggestions:

Realized the TAB solenoid I swapped out earlier last year was actually the wrong part. Replaced with the correct Motorcraft OEM part, also took apart the smog system at the diverter valve to verify the smog pump is indeed working and the diverter solenoid and valve are working properly. All checked out and after clearing the codes, installing the correct TAB solenoid and replacing the vaccum lines in the area, took it for a drive and got a KOER code 11. So no electrical/mechanical issues the computer is seeing.

BUT still getting the vibration below 1500 rpm that it’s had ever since I got it. To refresh, balancer has been replaced, motor and trans mounts replaced.

Once I get some mineral oil to hook up the smoke tester I got, I’ll check the vacuum system, but assuming that checks out okay, what other areas are there to look at?

I’m thinking possibly the trans crossmember bushings, or the flywheel? PO replaced clutch/flywheel back in 2014, and I bought the car in late 2018. No idea how hard he drove the car in that time, though from what I can piece together he didn’t put more than 10-15,000 miles on it during that time. I’ve had no trouble shifting gears or any other transmission issues, and the vibration occurs whether the trans is in gear, or in neutral, clutch in or out, doesn’t matter. So maybe I’m barking up the wrong tree.

Have also read that the PIP sensor in the distributor can cause a vibration since it will cause timing issues. As far as I know, only the cap and rotor have ever been replaced and not the full distributor. I have a new one I can put in, but not able to go through and set new timing afterwards so I didn’t want to get into that.

Does the vibration change at all with the clutch in or out?

I've been dealing with a hesitation while driving around town, about 45 mph or less. Does your car have a surge/hesitation while driving?

And I fixed multiple things on my car. Found vacuum leak on lower intake right behind the distributor. Still seem to have a little leak on the bottom side of the throttle body where the spring is. Not sure if that can be fixed.

I also replaced motor mounts, transmission mount, balancer, driveshaft u joints and had the ECU repaired.

I have new motorcraft o2 sensors I have to put in. But at this point it's quite frustrating.

Most of the stuff I replaced needed replacing, except for the u joints.

Mstng93SSP said:

Does the vibration change at all with the clutch in or out?

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I went for a brief drive today, checked while in motion and at idle. Clutch in or out made no difference, and the vibration occurred at the same RPM either way.

91GTstroked said:

I've been dealing with a hesitation while driving around town, about 45 mph or less. Does your car have a surge/hesitation while driving?

And I fixed multiple things on my car. Found vacuum leak on lower intake right behind the distributor. Still seem to have a little leak on the bottom side of the throttle body where the spring is. Not sure if that can be fixed.

I also replaced motor mounts, transmission mount, balancer, driveshaft u joints and had the ECU repaired.

I have new motorcraft o2 sensors I have to put in. But at this point it's quite frustrating.

Most of the stuff I replaced needed replacing, except for the u joints.

Click to expand...

That would be quite annoying. Mine does not have surge or hesitation.

That being said, I’ve replaced nearly all of the sensors, including the O2. The new O2 sensors did make the engine run slightly smoother, and improved fuel economy back to factory fresh. I attribute that to them being new Motorcraft sensors. A previous owner evidently had Bosch sensors in there. The improvement was immediate so definitely recommend swapping those out ASAP, since the computer uses that info to help control fuel/air ratio.

I’ve also gone through and used electronics cleaner in all the connectors I’ve touched in the process. Some had noticeable light corrosion or remnants of dried up dielectric grease inside the connecting surfaces, which can interfere with signals. I’ve cleaned the MAF using MAF cleaner, and cleaned the throttle body and blade as well. All of these have helped my car run slightly better, though none have been a cure all on their own. I’ve also just gone through and checked all of the engine grounds and surfaces.

if you haven’t already, check out the surging idle checklist and go through it. A lot of what I’ve done above is in there, and I’ve done it just for peace of mind and preventive maintenance, even if it wasn’t absolutely necessary.