Good Morning/Afternoon/Evening everyone! I'm a new fox owner as of about 6 months now. I have an 89 GT 306 with a generous list of all your common upgrades such as GT40 heads, E303 cam, BBK CAI and 70 mm throttle body, Trick Flow upper intake manifold, SCT tune and I'm sure I'm missing alot but you get the point. The engine was stripped and rebuilt about 1,500 miles ago and the T5 trans was rebuilt with hardened internals as well. My issue is around 1900 to 2400 rpm, she tends to buck pretty violently and has pretty sporadic rpm jolts. I put new plug wires in, swapped to a new $360.00 MSD Pro Billet distributor, never changed the plugs themselves though as I'm assuming they're new with the engine rebuild. I've also played around with the timing anywhere from 10 to about 18 degrees to see if that helped at all but no luck. I've replaced the TPS and made sure it was dialed in to about .950 to 1.000 every time I fiddle with it. I even ran smoke tests on it to eliminate any vacuum leaks I found. I'm running thin on ideas so far so any input would help as I am a first time Fox owner. My idle is great and at WOT she pulls like a champ. Also to note....its naturally aspirated so no SC or turbo added. Sorry for such a long thread just looking for help before I break the bank and take her to the shop!
The ever so common bucking issue!
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This is the famous 'surging idle checklist ' put together by jrichker and it will show you how to properly diagnose idle, stall, bucking and puking problems, well maybe not puking but it helps with everything else, 'but my car is not doing that' you say? Well it will help you fix the bucking problem too, the proper procedures is key to these computer controlled cars, I will add that when you check your tps with the volt olm meter check that the sweep has a steady rise to max volts, open the throttle slowly to make sure there are no 'dead' spots in the sweep.The
Just start at the top of the list and do each step one by one, don't skip around.
This is the famous 'surging idle checklist ' put together by jrichker and it will show you how to properly diagnose idle, stall, bucking and puking problems, well maybe not puking but it helps with everything else, 'but my car is not doing that' you say? Well it will help you fix the bucking problem too, the proper procedures is key to these computer controlled cars, I will add that when you check your tps with the volt olm meter check that the sweep has a steady rise to max volts, open the throttle slowly to make sure there are no 'dead' spots in the sweep.The
Just start at the top of the list and do each step one by one, don't skip around.
whats wrong with me today? 
Hard to say. If you google search "E303 cam bucking" you'll see it's an issue that does occur time to time with the E303.
It's really a decision as to what's more cost effective. A chip and custom tune, or perhaps a new cam.
But before exploring any of those paths, I would rule out a mechanical issue by running through the surging idle checklist first.
It's really a decision as to what's more cost effective. A chip and custom tune, or perhaps a new cam.
But before exploring any of those paths, I would rule out a mechanical issue by running through the surging idle checklist first.
The tune needs 3-4* of timing pulled out in the low load area of the tune, the cam is not "in" yet and extra/ too much timing can cause the surge or bucking sensation.
The other scenario is that the AFR is too lean in that area but thats likely not the case, usually pulling power out of the engine with timing stops the bucking that is common with letter cams.
you can test this by setting the timing to around 24* and leaving the spout out and going for a drive.
The other scenario is that the AFR is too lean in that area but thats likely not the case, usually pulling power out of the engine with timing stops the bucking that is common with letter cams.
you can test this by setting the timing to around 24* and leaving the spout out and going for a drive.
You aint wrong there. Anything's worth a shot I suppose. Couldnt hurt to try it without
Just an update. 2000xp was right. Took the tune chip out, dialed in my idle and air fuel ratios, and she runs smoother than ive ever felt. Thank you guys so much for the help. Also had to dig a little into the surging idle checklist to better set the idle and tps in place so thanks for that as well.
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