Engine Problem Diagnosis

[GlennC]

Stock 93 mustang 5.0 with Explorer intake, Mustang fuel rails & injectors, computer. After revving or slowing down the engine shakes as the revs decrease. I also noticed a strange exhaust smell and the tips of the exhaust pipes (both sides) have a milk chocolate brown residue. No smoking and no apparent coolant loss. The engine idles fine and runs smooth at higher RPM and does not overheat. The engine mounts are tight. I've done the injector "listen" test & all seem to be clicking. I'm getting ready to pull the plugs to look for evidence of excess fuel or coolant in the cylinders. I thought the shaking might be due to the pressure plate or something else associated with the engine being out of balance but the shaking does not occur at higher RPM, only as the engine drops to low RPM.

Anybody run into this before?

Any suggestions as too where else to search for a cause?

Thanks

[gbranham]

Has the engine ever been rebuilt?

Greg

[bobl]

Has it always done this or did it just start? An out of balance often shows up just as you described. Rough in the 1000-2000 rpm range and then smooths out as rpm increases.

[narly1]

IMO the easiest and least invasive thing would be to clean the injectors to see if that resolves the problem. My theory is that they are partially fouled and only cause problems at low RPMs/during decelleration>

An ultrasonic cleaning in warm heated anti-freeze followed by the blow-out method in the video linked below is the way to go.

https://www.youtube.com/watch?v=gS9Q9TVGjUw

[GlennC]

Thanks for the replies. No the motor was not rebuilt but freshened. The car has about 125 miles on it. I don't recall the engine doing this at first but has become more noticeable all of a sudden. I forgot to mention in my post that the gas mileage is pretty poor. As far as the injectors, I plan to due a resistance test on them but considering the amount of effort involved to get to the injectors to test and clean the I might as well replace the injectors with new. I'm thinking of replacing the fuel regulator & MAP sensor too. I don't think it's a pump issue because as poor as the gas mileage is it must be getting more gas than it needs and the engine doesn't act like it's starving.

In response to Bobl, is the out of balance condition at low rpm caused by a fuel issue?

Also, I was wondering if using the Explorer intake with the Mustang injectors, fuel rail, computer, sensors, etc could be causing the issue? I heard of a lot of guys using the Explorer intake without any problem

[bobl]

Thanks for the replies. No the motor was not rebuilt but freshened. The car has about 125 miles on it. I don't recall the engine doing this at first but has become more noticeable all of a sudden. I forgot to mention in my post that the gas mileage is pretty poor. As far as the injectors, I plan to due a resistance test on them but considering the amount of effort involved to get to the injectors to test and clean the I might as well replace the injectors with new. I'm thinking of replacing the fuel regulator & MAP sensor too. I don't think it's a pump issue because as poor as the gas mileage is it must be getting more gas than it needs and the engine doesn't act like it's starving.

In response to Bobl, is the out of balance condition at low rpm caused by a fuel issue?

Also, I was wondering if using the Explorer intake with the Mustang injectors, fuel rail, computer, sensors, etc could be causing the issue? I heard of a lot of guys using the Explorer intake without any problem


No I'm referring to a physical imbalance. If it just started then that's not the problem.

[TxMike64]

Check for oil on the plugs and replace the PCV valve.

[Presdough]

The chocolate brown exhaust is actually a sign of a correct air/fuel ratio. Rich would be black, lean would be light gray. I would look for a local vacuum leak affecting a single cylinder. It would smooth out at high RPM as the leaked air would be a smaller percentage of the total volume. Short of that I'm thinking balance. Were any rotating parts replaced when the engine was freshened up?

[J R Jones]

IMO the easiest and least invasive thing would be to clean the injectors to see if that resolves the problem. My theory is that they are partially fouled and only cause problems at low RPMs/during decelleration>

An ultrasonic cleaning in warm heated anti-freeze followed by the blow-out method in the video linked below is the way to go.

https://www.youtube.com/watch?v=gS9Q9TVGjUw

I have been freshening a V6 Acura engine that ran well. I had the injectors out and tried cleaning them myself using a similar wire harness, detergent and a fuel pump for pressure rather than a spray can and neglegable PSI.
I had acceptible spray patterns but two injectors did not shut off. This inevitably caused the fuel rail pressure to drain into the intake and open combustion chambers. I replaced all the injectors.
jim