00 Gt Surging / Bucking Under Load. Easy To Demonstrate Symptom On Freeway.

Is there a common area to troubleshoot for surging or bucking under load? When I say surge/buck it feels like the engine is briefly losing power and then back to normal but this symptom is occuring 3-4 times per second under the right conditions.

for example when on the freeway in 5th gear I can easily produce the symptom by pressing the throttle about halfway, the more gas I give it the more severe the symptoms.

Also when accelerating from a stop light the surging occurs as I'm accelerating through 1st and starts to smooth out as I reach the upper rpms of 1st, shift into 2nd and it might surge alittle and then smooth out etc.

The symptom is almost non existant when the car is cold but soon as the temp gauge needle rests at its usual warmup spot the symptoms will get worse.

Check engine light hasn't come on and the symptoms have been there for over a year but it only recently got noticabley worse to where the car isn't fun to drive in it's current state.

I've replaced the fuel filter with a napa gold filter but this didn't help,I also replaced a coilpack that was throwing a misfire code but it only helped a little bit, I pulled the sparkplugs and they all looked normal from what I could find online.

Ive also checked for smoke from tailpipes but not seeing anything, even after letting the car sit for 3-4 days and then fire it up, nothing from tailpipes.

Engine fires right up everytime with no difficulty and idles normally.

The car still has 4 original COPs installed(car has 106k miles) I haven't had a lot of money to throw random parts at it but I'm guessing replacing the COPS would be a good start?

I'm a little hesitant that COPS will solve the issue only because the symptoms seem different from when I've had bad COPS in the past, but I could be wrong.

Engine is stock, stock air intake with K&N filter.

Any ideas appreciated.
 
  • Sponsors (?)


The first thing that you need to understand in order to solve this issue is that there isn't one single cause for bucking. Sometimes there are multiple reasons for bucking. There are some causes for buck/surge that are more common than others.

If trying to solve this problem on a budget it's going to be necessary to TEST for a ton of possibilities and avoid the urge to "throw parts" at the problem.

The other key to solve this problem is to start with the basic maintenance items. Are all basic tune up items up to date?

How about the battery and charging system. Does the battery hold a charge? Are the battery terminals clean and tight? Has the alternator been tested for correct output and excessive AC ripple? I'm suspecting that you won't believe that bucking "could" be caused by something as simple as a bad alternator diode.

The Ford service manual has a long section on how to trouble shoot bucking/surge issues. If interested in getting a copy for yourself I maybe able to help. PM if interested.

It lists some "possible" causes as:
  • excessive EGR flow
  • MAF (when was the last time it was cleaned?)
  • secondary ignition (read COP or spark plug insulator boots). Add to this list timing (read timing chain ware).
  • fuel delivery (read incorrect/unstable fuel pressure or clogged injectors)
  • exhaust
  • PVC system
  • EVAP system
  • automatic transmission
  • base motor (read low compression or excessive leak down)
  • Get this idea?
Note. Charging system is not on the list because the Ford service manual was written for a professional mechanic. No professional mechanic would even start the trouble shooting without reviewing the condition of the charging system.

If looking for some general "short cuts", if the problem happens mostly under higher load and lower RPM's, then think:
  • Excessive spark plug gap
  • secondary ignition problem including insulator boot break down allowing spark to escape.
  • Insulation breakdown inside the COP
  • Excessive EGR flow.
Based upon your opening post, if you are certain that the ignition system is in good shape, then focus on EGR. Why? Because there's a good chance that the PCM will be calling for EGR flow under the lower RPM cruise driving condition.

So how do you rule out/in excessive EGR flow? The low tech way is to block of the EGR line or disconnect and plug the EGR valve vacuum line (note this won't provide a conclusive test if the EGR valve is leaking).

The other way is with an ODB2 scanner. It will need to be a higher end scanner that will access the EGR flow PID's.

Here's some information on a Windows based affordable ODB2 scanner.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

The data graphing capabilities could prove vital to solving this problem without throwing a crap ton of parts at it.

But as stated early, start with the basics. Don't cut corners. Especially with the charging system and other regular maintenance items.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

Note. No where in this reply has the word "replace" this or that been used. Test first. Then repair. Replace only as a last resort.

Regarding your question about replacing COP's. Instead of replacing the COP's replace the COP boot insulator. Autozone makes a full set of 8 replacement COP boot insulators that cost less than the price of a single COP.
 
Last edited:
  • Like
Reactions: 1 user
wmburns thank you very much for the detailed response, It's been about a year since I checked the spark plugs (when the symptoms first started occuring) and the plug gaps were correct as well, the symptoms were very minor back then compared to how its behaving now (surging much more pronounced now / passengers can easily notice it) I think I'll start with disconnecting & plugging the EGR vacuumm line, I have a cheap scanner that can reset the CEL after reconnecting EGR (assuming cel will come on).

It's been several years since I cleaned the MAF but I do have a can of maf sensor cleaner and will also try that, I may as well pick up new COP boots as the 4 original COPS are a little questionable (small cracks at the bottom of the boots).

You've definately given me a lot to work with, i'll report back my findings.
 
The confusing part (and reason I delayed my response) this bucking / surging symptom is worse on some days than others, The day I wrote my opening post it had the worst day yet but then the next day (I hadn't tried any troubleshooting steps yet!) it was actually difficult to make the car surge(?) even on the freeway in 5th gear it surged only one time (that I could feel).

The only reall difference was the weather, the day when it was really bad it was 110F out and the next day when I drove it on freeway it was around 90F.

So far I've tried plugging the EGR vacuum line, hopefully I did it right, I removed and plugged the vacuum line located at top of EGR valve and left everything else as is, (this didn't trigger CEL)

I also cleaned the MAF sensor, but with the surging symptoms reduced for unclear reasons It's hard to say if trying these steps helped at all, I can say it doesn't feel any different and the one symptom that is consistently there before & after the egr / maf steps is hesitation when taking off from a stop, it smooths out as 1st gear winds out and then is usually fine through rest of the gears, if I give it a minor rev before taking off I can sometimes avoid this hesitation.

I think I'm dealing with 2 different symptoms here but maybe they're linked, if I can figure out why it stumbles on takeoff.. the hesitation is also very minor or non existant within the first 5--7 minutes of driving the car (when engine is cold) it gets worse after warmup.

But next I plan on replacing the old COP boots just to rule that out.

What a pain, I just want my smooth running 4.6L back! lol
 
Just a short note to remind you of a few details. The PCM will behave very different under part throttle than a Wide open throttle. During WOT the PCM:
  • Disables the AC
  • Disables EGR
  • Adds fuel
  • adjusts timing
Also note about the EVAP system. The EVAP purge will generally happen within 15 minutes of driving AFTER a 6+ hours cold soak. So if looking for a "possible" reason why something might be different at the start of some trips verses other, an EVAP purge cycle might fit the bill. The list of conditions needed for an EVAP purge to run (or not run) is long enough to be a novel. The gas tank can't be too full or too empty. It can't be too hot or too cold outside.

Regarding disconnecting the EGR vacuum line setting an EGR code. I suspect that it will sooner or later. Remember that for the PCM to set an EGR related DTC you have to be driving the car during conditions where the PCM would be calling for EGR in the first place.

Outside temperature can affect how the motor runs. So can the blend of gas (winter vs summer).