74 914 2.0 dJet slight hesitation |
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74 914 2.0 dJet slight hesitation |
nsargeant |
Mar 12 2020, 07:38 AM
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#1
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Newbie Group: Members Posts: 12 Joined: 27-August 17 From: Vincennes, In Member No.: 21,381 Region Association: Upper MidWest |
In the quest for a perfect running 914 im getting oh so close. In the past I have replaced the tps with a new one that fixed the severe bucking/hesitation. The car ran so much better after that. What I am trying to figure out now is a random hesitation/hiccup that happens mostly during steady throttle when I am cruising on a highway. If I drive 30 miles it might happen once or twice. It seems pretty minor compared to the bucking I was having with the old tps. If I recall it has had this hesitation since I have owned the car for the last 3 years. I know the distributor does have a pertronix module in it. Ive recently changed replaced a few things listed below but still have that little hiccup. What would you guys investigate next?
Replaced: Fuel pump/filter Installed SS lines plugs, wires, rotor, (cap still looked new) tps sensor |
BeatNavy |
Mar 21 2020, 12:20 PM
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#2
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Certified Professional Scapegoat Group: Members Posts: 2,924 Joined: 26-February 14 From: Easton, MD Member No.: 17,042 Region Association: MidAtlantic Region |
A failing MPS diaphragm that doesn't hold vacuum well (or at all) would normally result in a rich condition. I still think your hiccup is most likely caused by something going intermittently lean from interruption in fuel delivery (e.g., fuel pump briefly stopping, injector skipping a cycle, etc.) or a momentary interruption in ignition (e.g., no spark briefly).
FWIW, I believe the real test of a valid MPS in terms of vacuum is something like: pump it up to 15 psi, and it shouldn't leak down below 10 psi in less than a minute, and it shouldn't leak below 4 psi in less than 5 minutes. Or something like that. Think of it this way: higher pressure (e.g., 15 psi) is associated with a closed throttle. The manifold has lots of pressure as the engine is running and pumping air, but the throttle is closed. That's an idle situation requiring less fuel. A wide open throttle has very low pressure in the manifold (e.g., 0 psi), and that relates to the need for much more fuel. Part load is somewhere in between, usually measured at 4 psi. Overrun is higher than idle psi (e.g., 18 psi) with no real requirement for fuel. The hole in the back of the MPS is someone who was attempting to tune that MPS, probably to make it richer. That may or may not be related to the root cause of what you're experiencing. |
Bleyseng |
Mar 21 2020, 12:51 PM
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#3
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Aircooled Baby! Group: Members Posts: 13,035 Joined: 27-December 02 From: Seattle, Washington (for now) Member No.: 24 Region Association: Pacific Northwest |
A failing MPS diaphragm that doesn't hold vacuum well (or at all) would normally result in a rich condition. I still think your hiccup is most likely caused by something going intermittently lean from interruption in fuel delivery (e.g., fuel pump briefly stopping, injector skipping a cycle, etc.) or a momentary interruption in ignition (e.g., no spark briefly). FWIW, I believe the real test of a valid MPS in terms of vacuum is something like: pump it up to 15 psi, and it shouldn't leak down below 10 psi in less than a minute, and it shouldn't leak below 4 psi in less than 5 minutes. Or something like that. Think of it this way: higher pressure (e.g., 15 psi) is associated with a closed throttle. The manifold has lots of pressure as the engine is running and pumping air, but the throttle is closed. That's an idle situation requiring less fuel. A wide open throttle has very low pressure in the manifold (e.g., 0 psi), and that relates to the need for much more fuel. Part load is somewhere in between, usually measured at 4 psi. Overrun is higher than idle psi (e.g., 18 psi) with no real requirement for fuel. The hole in the back of the MPS is someone who was attempting to tune that MPS, probably to make it richer. That may or may not be related to the root cause of what you're experiencing. I agree, look elsewhere for this problem. (IMG:style_emoticons/default/headbang.gif) |
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