'97 4.0 OHV idle shudder problem

[jayhawkexplorer]

Hey guys,

I recently had a CEL problem which turned out to be an O2 sensor. Yes, for once replacing the O2 sensor actually fixed the problem. But now, its having an idle issue without any CEL. Whenever it idles, even after its warm, there's a noticeable shuddering of the engine. It happens in P or if I'm at a stoplight in D with my foot on the brakes. When i first start the truck up, it shudders horribly in D and R, up utnil around 15 MPH. After its warmed up, whenver I give it a little gas, even as little as 13-1400 RPM's, the shuddering stops. It feels like the truck has an air/fuel problem at idle. The mileage and performance are still normal and the truck doesn't ever completely die, but it feels like its going to and I'd rather not have it happen in the middle of downtown traffic someday soon. Here's the problems I'm thinking it could be:

1. IAC valve (replaced last year so I don't think that's it)
2. MAF sensor (haven't ever done anything to it, strong possibility)
3. Fuel filter (hasn't been changed in a while)
4. Fuel pump (just a guess, no logic behind it)

Any ideas?

 

[jayhawkexplorer]

And tonight on the way home, at a stoplight, it started making this awful rattling noise but it went away within a couple blocks. I think its going to have to go to the dealership soon; I think this repair might be beyond me. Anybody have any ideas??

 

[jayhawkexplorer]

bumpity bump bump

 

[dpirog]

Are the rpm's dropping when you feel the shutter?

 

[jayhawkexplorer]

Not visibly on the tach but I can feel the vibration, it feels like its running out of gas. Not being any kind of expert mechanic, I think it might be the fuel filter. Unless I give it throttle, it feels like its running out of gas. When I hit the throttle, it forces gas through the filter and makes it run ok. Is this completely illogical or am I maybe on the right track?

 

[Lazzman]

I have a '98 with the sohc V6 and it does the exact same thing. Cannot figure out what the hell it is. Dealers and others seem to think it is the IAC but I replaced it twice with a NAPA model, no change.

I am thinking of buying the Motorcraft one but don't want to waste money.

My rpm's do drop down to nearly zero and then catch and bounce back up to 600rpm or the truck just stalls out. Sometimes they just drop a couple of hundred rpms and catch.

It feels serious as the whole engine shudders violently. Wish I knew what it was. Idle does not kick up in park as it should when accessories are turned on such as A/C.

Would be very grateful for advice?

 

[dpirog]

I had a similar issue. Rough idle but no visible drop in rpm on tach. Turned out to be weak motor mounts and a very weak tranmission mount (the tranny mount rubber had actually collapsed leaving metal to metal contact). The vibration from motor/tranny ass'y was being transmitted to the cross member under the tranny and shaking the whole vehicle (I have an a/t, not sure if m/t's mount differently).

A mechanic found this for me and verified the diagnosis by pushing up on the tranny tail while idling. Whole thing smoothed out while the tranny tail was lifted up in the mount.

Might not be your problem but it's easy to check.

 

[jayhawkexplorer]

Interesting, that's going to be what I check first thing tomorrow morning. Thanks, Boomer Sooner!

 

[Viper81]

my dad had an old cutlass and it did that shuttering at idle thing. he replaced the fuel filter and it ran like it was brand new. plus your is a '97, it couldn't hurt to do that. it's like an hour job max.

i also had a contour where the motor mounts were completely wore out and got those replaced also but it didn't lower the rpm's. all that did was smooth out the ride and some shifting problems i had with that.

but my first check would be the fuel filter.

Viper

 

[jayhawkexplorer]

Yeah, that's what I'm thinking too Viper, after looking through the Haynes manual and seeing what's involved in checking tranny/engine mounts. Not something I can really handle in my apartment carport. But the fuel filter I can.

 

[bluexplorer1]

Hey guys. I have a 4.0 SOHC with the same kind of problem. In drive I have a shudder in my idle that can be felt in the steering wheel. Its almost like an intermittent pulsating. Anyway, if I am sitting at a stop light with my foot on the brake and I give it a little bit of gas, the idle smooths out. If I give it gas for too long though, the RPMs drop down to about 350-400 and it almost stalls out. I heard this could be caused by a bad powersteering pump. Any one have any ideas on this?


Brad

P.S. My fuel filter is only about 10k miles old so I doubt that is the problem.

 

[jayhawkexplorer]

I just got the fuel filter changed out and took it for a drive. It seems like the problem has lessened but may not totally be gone. The real test will be tomorrow when its been sitting all night, that's when the entire driveline shudders at low speed. *FINGERS CROSSED*

 

[jayhawkexplorer]

Ok, the fuel filter was definitely not the problem. It shuddered and shook as bad as ever today - maybe it is that motor mount thing dpirog suggested.