Engine Bad vibrations

paddyrk · Jun 12, 2019

I have a 1991 mustang with a 347 and the hp efi system, and I am having terrible vibrations. The engine seemed to have lost power on the highway yesterday, I tried to floor it and it felt like I was at 1/3 throttle. At idle and under load during low rpm, there is terrible vibrations. The cl comp seems to be okay, although on my morning commute I would see it around zero % at cruising, but sometimes it would go to -25+% while under the same condition. If anyone has anyone ideas, or anything to check, please let me know, I checked the transmission fluid and did a visual check this morning but nothing was out of the ordinary. I feel like the motor mounts may be to blame, but I am unsure what would cause them to ‘fail’ so suddenly. Thanks

General karthief · Jun 12, 2019

Your vibration and loss of power could be related. Does the hp system (I don't know what that is) use your computer so you can check for codes or does it have it own system check mode?

paddyrk · Jun 12, 2019

General karthief said:
Your vibration and loss of power could be related. Does the hp system (I don't know what that is) use your computer so you can check for codes or does it have it own system check mode?

It’s is a Holley stand-alone ecu, and there’s no check engine codes or anything like that, but I can see everything from afr, map pressure, timing, fuel flow etc.

General karthief · Jun 12, 2019

Can it data log?

a91what · Jun 12, 2019

Get me a datalog to look at, check all the injector clips as well.
If there is an issue I will see it. Also check the cap,rotor, plugs ect... a missfire will look like a lean spike.

paddyrk · Jun 12, 2019

a91what said:
Get me a datalog to look at, check all the injector clips as well.
If there is an issue I will see it. Also check the cap,rotor, plugs ect... a missfire will look like a lean spike.

Will do, it will come as a Holley efi data log

a91what · Jun 12, 2019

Thats ok I should be able to open it up as a delimited file in MegaLogViewerHD

Mustang5L5 · Jun 12, 2019

My initial guess is a misfire or dropped cylinder. You can run on 7 cyl with these cars and just think you have a minor vibration

paddyrk · Jun 12, 2019

Here are my data logs, one starting when I idled in park then went for a drive and parked, and the second one is me starting the car after I parked it after the drive.

I pulled all the spark plugs as well beforehand, and found one that was somewhat loose in the head, and it was still wet and reeked of gas, but when I held a flame to it, it did not catch. Other than that plug, the rest looked like this.

Anything helps, thanks.

paddyrk · Jun 13, 2019

Mustang5L5 said:
My initial guess is a misfire or dropped cylinder. You can run on 7 cyl with these cars and just think you have a minor vibration

What would be a good way to test this? My motor has about 4k miles on it if that, would it be the basics like compression, spark, and fuel? And it's not just minor, when I give it throttle, it feels like I have massage seats.

General karthief · Jun 14, 2019

You need to channel your inner mechanic because it could be as simple as a bad plug/wire, cracked distributor cap to a head gasket failure between two cylinders. Failed motor mount(s), The list goes on.
did you buy it modified ?

paddyrk · Jun 14, 2019

General karthief said:
You need to channel your inner mechanic because it could be as simple as a bad plug/wire, cracked distributor cap to a head gasket failure between two cylinders. Failed motor mount(s), The list goes on.
did you buy it modified ?

No, I bought it stock except for rear seats and a coil, then did my own 347 swap, replacing nearly everything, I’m replacing things tonight in this order until one fixes them: plugs, distributor cap, coil, o2 sensor.

a91what · Jun 14, 2019

O2 sensor likely not the issue, I have not looked at the log file yet.
I had this same issue when one of my injectors stopped working, car felt terrible. Turns out it was a bad connection due to a broken tab on the injector clip.

paddyrk · Jun 14, 2019

a91what said:
O2 sensor likely not the issue, I have not looked at the log file yet.
I had this same issue when one of my injectors stopped working, car felt terrible. Turns out it was a bad connection due to a broken tab on the injector clip.

Okay, I was thinking of doing an injector test, but did not want to do unnecessary work (except I still own a fox body), the best way is to pull the rail off the intake, leave injectors connected to the rail, give each one its own cup, and turn the engine over with the distributor disconnected, correct?

a91what · Jun 14, 2019

you can do it that way, I took the upper off and used a boroscope to look in the runner.

The MS systems can do an I/O test, so i just fired the injectors one at a time to find the culprit.

paddyrk · Jun 14, 2019

After replacing the coil, plugs, rotor, and cap, the engine started much easier and ran better, but it still misses pretty bad, I used my timing light to make sure all the cylinders were getting spark, and sure enough, cylinders 2&3 were not receiving any. I did a visual inspection, changed spark plug wires, even went back to the previous cap, and nothing gave them any spark. All the other cylinders get plenty of spark.

What could cause spark to only 6 of the 8 cylinders? I am at a loss.

General karthief · Jun 15, 2019

Swap a couple plug wires around and see if the problem follows the wires, if not then swap plugs.

paddyrk · Jun 15, 2019

General karthief said:
Swap a couple plug wires around and see if the problem follows the wires, if not then swap plugs.

I swapped the wire for plug 2, and I replaced all the plugs yesterday too, but I will try more with the wires.

paddyrk · Jun 16, 2019

I have a 1991 LX V8 with an aftermarket ECU, and I am missing spark on cylinder 3. I used my timing light to verify that it was in fact not sparking, and that the other cylinders were, and after pulling the plugs, the other three on the passenger side bank were running rich, and the cylinder missing spark (number 3) was wet and reeked of gas. When I used the timing light on it, it was like the spark was syncopated, a few sparks, nothing for a second or two, a spark here, a spark there, but no consistency whatsoever. When I run the engine, it vibrates more than usual at idle and at cruising RPM, but when under load, it sounds bad, has no power, and shakes a lot, especially at low rpm when I give it some throttle. I have replaced the coil, rotor, cap, plugs, and the wire of the bad cylinder. I switched plug 1 & 3 and the problem persisted, and I switched wire 1 & 3 and the problem persisted.

Side note: the idle sounds so weird it almost is like a rotary when I am in gear.

Any thoughts? Any input helps, thanks.

General karthief · Jun 16, 2019

You already have a thread on this problem?

Engine - Bad vibrations

I have a 1991 mustang with a 347 and the hp efi system, and I am having terrible vibrations. The engine seemed to have lost power on the highway yesterday, I tried to floor it and it felt like I was at 1/3 throttle. At idle and under load during low rpm, there is terrible vibrations. The cl comp...

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Engine Bad vibrations

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That is…until I whipped out my Bissell

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wonder how much it would cost to ship you a pair

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SendMeUrDataLog

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wonder how much it would cost to ship you a pair

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wonder how much it would cost to ship you a pair

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