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Topic: Help! Terrible idle surge (Read 779 times) previous topic - next topic

Help! Terrible idle surge

I've had this idle surge/rolling idle ever since the upper ho conversion was done. Map sensor was not changed, but another was swapped and the problem was still there. The part #'s were checked by a Ford dealer and crossed for an 87 ho. However, vacuum lines were kept original but there don't appear to be any vacuum leaks. New felpro intake gaskets were used with a gasket cinch type sealer. Tps is .9 volts.
 The ho cam was retarded 4 degrees in an attempt to increase the engine rpm for the blower boost. Imo, the retarding of the camshaft may be causing the intake to close too late causing the reversion to pump some of the charge back into the intake reducing the vacuum and playing heck with the map. Am I out of the box on this?
 Engine vacuum is 14" when it starts to surge the vacuum goes back and forth from 14" to 11" and the rpm from 600-800. I can't stand driving it when it acts up.  :punchballs:    Any insight or advise is appreciated  ;)

 Thanks,
 Mike
[SIGPIC][/SIGPIC]88 t-bird: 5.0ho, gt40y, crane 2031, fms 1.7, paxton@5#, aod wide ratio, tci stall, performer rpm upper, 70mm bbk, pro m 60, 42#s, 3.73 7.5" posi, jba shorties, borla, upr x. 13.4 @ 104mph. cbaza, moates, tuned by decipha

Help! Terrible idle surge

Reply #1 –
SD needs more than 14" vac to operate properly(min of 16/17")... If you don't have any vac leaks, guessing you need to advance the cam back to specs...

Increasing the idle should help... Even with Mass Air, I keep mine at 950 rpm IN GEAR, got tired of the sumbich stalling at inopportune moments..

Help! Terrible idle surge

Reply #2 –
Quote from: TurboCoupe50;159460
SD needs more than 14" vac to operate properly(min of 16/17")... If you don't have any vac leaks, guessing you need to advance the cam back to specs...

Increasing the idle should help... Even with Mass Air, I keep mine at 950 rpm IN GEAR, got tired of the sumbich stalling at inopportune moments..


Thanks turbocoupe50
 Someone in a supercharger forum posted that speed density doesn't give a rat's a$$ about vacuum leaks. Is there any truth to that and if there is a vacuum leak, how large do you think it would have to be? Would a higher idle result?
 I will clear the idle strategy, reset the idle and change some vacuum lines 1st.
 Thanks for the advise
[SIGPIC][/SIGPIC]88 t-bird: 5.0ho, gt40y, crane 2031, fms 1.7, paxton@5#, aod wide ratio, tci stall, performer rpm upper, 70mm bbk, pro m 60, 42#s, 3.73 7.5" posi, jba shorties, borla, upr x. 13.4 @ 104mph. cbaza, moates, tuned by decipha

Help! Terrible idle surge

Reply #3 –
SD won't care IF the leak isn't so much the vac level starts to fall below 16" or so as I mentioned..l... It will defiantly cause problems if the vac is low to begin with... There's no wonder it's surging at 14"...

When you increase the idle don't forget to reset the TPS voltage, as it will likely increase to over 1v...

BTW If you have a hand vac pump connect it to the MAP and pull 17"/18", it should idle good if there are no issues other than the cam retard..

Help! Terrible idle surge

Reply #4 –
Quote from: TurboCoupe50;159469
SD won't care IF the leak isn't so much the vac level starts to fall below 16" or so as I mentioned..l... It will defiantly cause problems if the vac is low to begin with... There's no wonder it's surging at 14"...

When you increase the idle don't forget to reset the TPS voltage, as it will likely increase to over 1v...

BTW If you have a hand vac pump connect it to the MAP and pull 17"/18", it should idle good if there are no issues other than the cam retard..


That's what's kinda drive'n me nuts, with such a low vac one would think the leak would be somewhat obvious.(It didn't have egr or iac codes)
 If it did turn out to be cam timing related, that factor could be valueable for speed density cam selection, but it's probably an overcomplicated long shot :hick:
 The idea of the hand vac pump to the map @ idle is great ;)
 I'll try to post the results

 ps. You got yourself a quick car even without the juice  :evilgrin:
[SIGPIC][/SIGPIC]88 t-bird: 5.0ho, gt40y, crane 2031, fms 1.7, paxton@5#, aod wide ratio, tci stall, performer rpm upper, 70mm bbk, pro m 60, 42#s, 3.73 7.5" posi, jba shorties, borla, upr x. 13.4 @ 104mph. cbaza, moates, tuned by decipha

Help! Terrible idle surge

Reply #5 –
Quote from: t.birdsc;159475


 ps. You got yourself a quick car even without the juice  :evilgrin:


Thanks, but I hope to make it a little quicker IF I ever get the 331 finished... Would really like to reclaim #1 NA street...

Help! Terrible idle surge

Reply #6 –
Quote from: TurboCoupe50;159498
Thanks, but I hope to make it a little quicker IF I ever get the 331 finished... Would really like to reclaim #1 NA street...



Since you already ran a 12.23 on the juice, the 331 on the motor should put u into that range every thing else being equal. People will be chasing that for a while. Low 12's on the motor. :evilgrin:
[SIGPIC][/SIGPIC]88 t-bird: 5.0ho, gt40y, crane 2031, fms 1.7, paxton@5#, aod wide ratio, tci stall, performer rpm upper, 70mm bbk, pro m 60, 42#s, 3.73 7.5" posi, jba shorties, borla, upr x. 13.4 @ 104mph. cbaza, moates, tuned by decipha

Help! Terrible idle surge

Reply #7 –
Well I doubt it would run low 12s, but if it gets near 12.5 I'll be more than happy... It will take a few other changes to do it, but that's the plan...