Engine shake at idle

G Man

New Member
Dec 4, 2020
14
1
3
51
Bay Minette, Alabama
Hi. I have a 90 5.0., and I have an idle problem I cannot resolve...

The car is an automatic. It has about 75,000 miles on it. The engine is all stock except for headers, and I removed the smog pump. The car runs strong, burns no oil and displays no peculiarities of operation except for the idle issue, which is as follows:

The engine has a slight jerk or judder that is immediately noticeable at idle. You can really feel it when you put your hand on the intake, and you can feel it in the steering wheel at idle. If you listened to the car it would sound fine and idles steady at 750 rpm and holds 18 inches of manifold vacuum.



To fix this I have tried the following:
New Motorcraft IAC
New Motorcraft TPS
New Mallory coil
New gapped plugs
New wires
New cap and rotor.

Any ideas?
 
  • Sponsors(?)


2000xp8

SN Certified Technician
Aug 8, 2003
7,117
1,176
194
NJ
If it's original, i'd replace the balancer.
They dry rot and can wreak havoc, even if it's not this problem, it still needs to go.
 
  • Agree
Reactions: 1 user

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
35,685
11,682
224
Massachusetts
Run the codes, perform the cylinder balance test. See if it can detect a misfire on one of the cylinders.
 
  • Like
Reactions: 1 user

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
19,461
6,479
193
polk county florida
I bet you have a plug wire arcing or the cap has a crack (more likely a plug wire) do the code check and the cylinder balance test.
 
  • Like
Reactions: 1 user

G Man

New Member
Dec 4, 2020
14
1
3
51
Bay Minette, Alabama
If it's original, i'd replace the balancer.
They dry rot and can wreak havoc, even if it's not this problem, it still needs to go.
Cars runs good. Car runs strong. Idles well. When you're in the car and have your handson the wheel it feels like someone in randomly bumping the steering wheel
 

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
35,685
11,682
224
Massachusetts
I have run the codes and there are none. I have a Pro-M processor, by the way
I was really asking to do the cylinder balance test, which is a primitive misfire detection program the stock ECU will run to see if an injector is dead or a cylinder is not contributing for whatever reason. I don’t believe the Pro-m can do this.

years back, I had a slight vibration at idke that disappeared above idle. I thought it was balancer. I ran the codes and got nothing useful. I ran the cylinder balance test and it showed cylinder 4 was dead. I had been running on 7 cylinders for a year and didn’t know it. It pulled hard and would spin the tires easy into 2nd gear....on 7 cylinders.

I ohmed out the injectors and sure as hell one was dead. Got 0 ohms wren I tested with meter. Replaced injector and all was good.

I still think you have a misfire, so you’ll need to troubleshoot the old fashioned way. Spark, fuel, compression. You can use a pressure guage on the rail and a 9v battery to trigger each injector. Prime the pump, get a static pressure reading and then trigger the injector one by one. It should drop.

you can pull a plug and lay on intake to look for spark.

conpression i would save for last, unless you suspect something like a hanging or broken valve, or a blown HG.
 
Last edited:
  • Like
Reactions: 1 user

G Man

New Member
Dec 4, 2020
14
1
3
51
Bay Minette, Alabama
Did a compression test. All cylinders were between 130 and 140 except #7, which was at 70. That was my problem. I just put this on here to finalize this thread and provide information for others in the future.
 
  • Like
Reactions: 1 user