Odd Engine Behavior Development

[DJ Fury]

2000 GT Vert 110k Miles. Not sure whats going on but this developed over the last couple weeks.

When I first start the car the engine idles normally. When I get on the road and come to a stop at say a light the rpms at idle dip to around 600 and I can feel a subtle shaking in the drivers seat that feels like its coming from the engine bay (engine shaking). I can feel the shake most noticeably at highway cruising rpm's (2000-2100 rpm). Much more pronounced when on the highway.

The other day when I was sitting at a light with my foot on the brake the engine surged for a second, got up to about 1500 rpm and then immediately dropped to like 500 and the Battery light momentarily lit up and the car felt like it would have died if I didn't let go of the brake. As soon as I lifted my foot the car surged forward a bit and then returned to normal. Only time this has happened in the life of the car and it hasn't done it since.

I'm thinking that the engine surge/battery light might indicate a failing alternator?? I have no clue about the engine shake at idle. Plugs, Oil, PCV Valve, Air filter all were replaced about 2 months ago.

Any help/suggestions?

 

[streethorse]

Have you ever changed out the IAC unit?

 

[DJ Fury]

I have actually. If I remember right I replaced that valve at my 80k mile mark. I can pull that off and take a look at it again though because I did replace it with a iac from autozone instead of one from ford.

 

[patman0911]

The battery light flashing when the RPMs dip just means the RPMs dropped low enough that it's not getting enough output from the alternator - if the RPMs drop low enough your oil pressure gauge will drop and some other indicators lights will flash. Don't worry about that, that's just a normal side-effect of almost stalling.

If the surge and dip was an isolated event, you may be able to chalk that up to, "sometimes weird crap happens with fuel injection when just the right set of circumstances and set of variables all line up just right and the moon is blue" but the shaking at idle and at 2K cruise may indicate that something is going on.

No loss of power at WOT?

Has the shake always been present at idle, or is this a recent development?

The shaking at 2K... does it shake in any gear while cruising at 2K or just at a certain speed? Need to make sure the shake at cruise isn't a suspension issue.

If it's doing it at idle and at cruise then my first guess would be a vacuum leak and/or a light misfire.

 

[DJ Fury]

I don't think this is one of those isolated events. The only difference I can tell at WOT is it takes longer for the car to respond to the pedal being floored where it used to plant ass to seat. Its like there's a couple second delay from when I floor it to when the car lurches.

The shake is definitely a recent development. I've owned the car throughout its 110k mile lifespan.
Yes it shakes in any gear at any speed around 2k rpm. You know now that you mention vacuum leak I've noticed the 2 preformed rubber elbow lines are starting to look the worse for wear. There's definitely cracks in both of those 90 degree elbows. One goes straight into the valve cover on the right side and the other is one that goes into the TB from the PCV valve on the opposite side.

I suppose I could try replacing those 2 elbows and see where that gets me but I know for a fact those preformed elbows are not sold on their own. I think you have to buy the whole assembly just to get the rubber elbows.

 

[wmburns]

I am going to assume this is an automatic transmission. If the lock up torque converter is not releasing as it should, this could case the problem.

When was the last time the automatic transmission was serviced? This may also explain the power loss at WOT.

Consider starting with a transmission service. At the least, check the fluid level and condition.

+1 on ignoring the battery light. It is from the engine running too slow.

And yes a trip to the dealer may be needed. And yes the parts will cost too much. But what are the options? If you could get the parts at a junk yard (not all that likely), they are likely to be worse than the ones you have. Further, what is your time worth?

These cars just will not run right with vacuum leaks.

Long shot, try the local auto parts store. Ask for the "help line" of parts. Sometimes they have universal fit elbows and such. Here's the rub, you may have to take it off of the car in order to attempt to match. Likely that it will come apart in your hand. I don't have to tell you what that means.....

The shake/vibration could also be broken/weak motor mounts. Not far fetched considering the age/mileage of the car.

 

[patman0911]

Duck Tape to the rescue! But, yeah, if you've got obvious cracking in those elbows, replace them.

If you've got a decent vacuum leak, then since not all of the air is coming through the MAF the computer may not be correctly calculating the amount of air expected for the accelerator pump-like squirt of extra fuel it should deliver when you step on the gas pedal. It may be delayed or may not be sized correctly and thus the slight delay between mashing the gas pedal and full acceleration.

Edit: You may not actually lose any power at WOT since with no vacuum in the manifold, the air is going to follow the easiest path and go in through the giant hole at the end of the intake tube and what little air is ingested through your leaks would just lean out the already rich A/F ratio at WOT a little.

 

[DJ Fury]

Well I stopped by Ford yesterday and sure enough you have to buy the entire tube that the elbows connect to. Damage wasn't that bad at about 25 bucks each assembly. Figure since I bought it and they're new parts I'll just put the 2 whole assemblies in instead of just the elbows. Won't get to it today as I have to go watch the Buckeyes stomp a mudhole. Probably be too wasted to work on the car today lol

Post back when I have everything installed and see how its runnin ..

 

[DJ Fury]

Update..Both have been installed for a few days now. The engine "shake" I was feeling has definitely lessened but it is still there. So at least those 2 vacuum elbows were part of the problem. I think I'll be replacing my IAC next..Sound good? I'm also considering replacing the motor mounts but have never done that myself so not sure I could pull that off on my own. How much does replacing the mounts usually run? Should I replace them with Urethane mounts?

 

[wmburns]

I paid a local shop $350 to change the motor mounts on my 1996 once. I supplied the mounts.

I recommended that you consider carefully with regards to Urethane Mounts. Expect increased Noise Vibration Harshness (NVH) with the hard Urethane mounts.

I may be going out on a limb here but you seam like the sort that likes a smooth running engine. No vibration. Just like a sewing machine.

There will be an increase in NVH with Urethane mounts.

I have owned three GT Mustangs (1996, 2000, 2003) and a 97 Cougar. All with the 4.6 modular V8. They all had a very slight occasional shake at idle (more like a bump). All of them were smooth as silk under power.

OBTW, I just replaced my IAC because it would sometimes die when cold (coasting to a stop). So far, so good. I have also noted that a slight idle surge is now gone.

 

[DJ Fury]

So stick with OEM motor mounts is what you're saying. Got it. Definitely like a smooth ride and other than a few exterior changes (headlights/taillights) my car remains bone stock. I'll not worry about motor mounts just yet. I'll be stopping by Ford to pick up an IAC next week.

 

[GOKBGO]

You haven't mentioned replacement of the fuel filter. Might want to clean the MAF sensor if never done.

 

[DJ Fury]

Actually have a replacement fuel filter I've had in the backseat for a month now. Been meanin to get it swapped just havn't had the time to get the rear end in the air. I'll probably get under there sometime this week.

 

[DJ Fury]

Ok quick question.. does it matter where I get an IAC from? I looked one up on my local ford dealer parts app and it was like 160 bux and autozone/advance have them for like 50. Anyone use these on their cars? Or would it be best to stick with Ford oem...

 

[wmburns]

My vote, Autozone. Have one installed on my car right now. Seams to work fine. I have had others installed on my other Mustangs. No problems.

With the $$ I saved, I can have my own set of Geico googly eyes.

 

[fshawn50]

yeah autozone one should be fine? If i remember i replaced mine with autozone one an its been fine?

So stick with OEM motor mounts is what you're saying..

I've priced them an a set from ford bout $85-$90 dollars oem rubber mounts life expectancy 6-7 years before rubber starts deteriating? an thats if your not hard on them.

Good set of Poly's ugh bout $135 an up? lil pricy? Last probly longer than you will have your car? didn't mean that in a hateful way ther.

An just changed mine out Saturday took bout 4-5 hours in the garage? me an a buddy? Saved that $360 plus install fee

An wish I had taken a pic. those things had split in two an were junked

An not really any more NVH than before when my motor was floppin around in the engine bay?