Q jet idle issue
#1
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Q jet idle issue
I've got the original Q jet (17080204) on my 1980 which now has a GM 290 HP crate engine (1995) coupled to a TKO 500 tranny. An RV cam with true dual exhaust is also on the car. Runs great, good fuel economy. Recently I began having an idle problem after coming to a stop. Car idles fine cold (electronic choke) and warmed-up, but the idle drops low when coming to a stop (nearly stalling). It recovers to normal (700 RPM) after a few seconds. Any thoughts? Also, is this the best model Q jet to use based on the mods that have been made? It seems to work fine, and I use manifold vacuum for the distributor advance. Thanks.
#2
what are you using distributor wise? Assuming it's an HEI or other electronic unit, check your timing first to make sure it hasnt changed. Make sure the carb. bolts havent loosened. Then I would just readjust the mixture screws to see if that resolves the issue.
#5
Drifting
Idle was fine; but when braking the idle would sometimes drop below 500 RPM and sometimes die. I was never able to bleed brakes properly until I bought a Motive Bleeder. During 1 test drive after successful bleeding the brake pedal was firm and the idle did not drop during braking. It must have had someting to do with vacuum, at least that's all I can think of. My car's braking/idle drop is exactly what the OP mentioned. I hope this helps...
#6
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Idle was fine; but when braking the idle would sometimes drop below 500 RPM and sometimes die. I was never able to bleed brakes properly until I bought a Motive Bleeder. During 1 test drive after successful bleeding the brake pedal was firm and the idle did not drop during braking. It must have had someting to do with vacuum, at least that's all I can think of. My car's braking/idle drop is exactly what the OP mentioned. I hope this helps...
Keep the shiny side up!
Scott
#7
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Never would have made that connection, but I did some brake work this winter and bled them. The brakes are fine, though the idle drop does occur during braking. I'll investigate further along with checking the carb bolts.
#8
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I can't see how air in the brake lines could have any effect on idle quality, directly. However, if there is a little air in the lines, and you are pressing down harder on the brake pedal, it would maybe make a small hole in the brake booster diaphragm open up enough to cause a bigger vacuum leak? I know, I know, it's reaching, but hey, anybody got a better guess?
Keep the shiny side up!
Scott
Keep the shiny side up!
Scott
#9
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Well, I did a little experimenting and found that the idle drop does occur during braking, not just throttle closure. I ran the vehicle up to cruising speed and pushed in the clutch; the idle went to normal (750RPM). When the brakes were applied it dropped to 400 and almost stalled.
I then tightened up the carb mounting bolts and repeated the test. The idle only dropped to 550 during braking. Seems like the vacuum-assist power brakes do affect the idle. I probably have a leak somewhere also. The Q jet is in need of a rebuild, and I've had the parts laying around for over a year. Time to get to work.
I then tightened up the carb mounting bolts and repeated the test. The idle only dropped to 550 during braking. Seems like the vacuum-assist power brakes do affect the idle. I probably have a leak somewhere also. The Q jet is in need of a rebuild, and I've had the parts laying around for over a year. Time to get to work.
#10
Drifting
What worked for me:
1 - Motive Bleeder - make sure you have hard pedal!
2 - Adjust idle mixture screws for highest vacuum @ idle
3 - Vacuum advance hooked up to full manifold vacuum
Good Luck!
1 - Motive Bleeder - make sure you have hard pedal!
2 - Adjust idle mixture screws for highest vacuum @ idle
3 - Vacuum advance hooked up to full manifold vacuum
Good Luck!
#11
Melting Slicks
Vacuum assist brakes + long throw = more vacuum stolen.
That said - I'd still expect to find a vacuum leak or issue elsewhere, and this is only making worse.
-W