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Finally wrapping up the 88 F-150, l6 to 351w swap. For reference its my first ground-up engine build. I understand, cam profiles are very limited by the speed density system, but I can't seem to get the engine to stop idle hunting. Runs great on cold start, but after a few minutes, the vacuum as well as rpm start to bounce between 12 and 18". Here are the build specs, cam card will be attached below. Any help is appreciated-
Short block is fully stock
3 bar GT-40 heads with upgraded trickflow spring kit
new stock 19 lb fuel injectors
351w intake port matched to a 302 upper/ plenum
1.7 ratio roller rockers (will removing these put valve lift back in compatible range?)
Comp Cams Hyd. Roller
Turn the idle stop screw out a little to open the throttle bores, see if that smoothes it out. Did you set the timing dot to dot, degree it in, advance it?
Last edited by 90project5.0; Jun 6, 2026 at 08:12 PM.
Turn the idle stop screw out a little to open the throttle bores, see if that smoothes it out. Did you set the timing dot to dot, degree it in, advance it?
timing was set at dot to dot, haven't touched the idle screw yet. Ignition timing is set at 12 btdc. Whats weird though and makes me thing its an issue with the speed density is that it runs flawlessly with the MAP sensor unplugged, of course at 1500 rpm though
timing was set at dot to dot, haven't touched the idle screw yet. Ignition timing is set at 12 btdc. Whats weird though and makes me thing its an issue with the speed density is that it runs flawlessly with the MAP sensor unplugged, of course at 1500 rpm though
Is your map sensor for a 5.8? Not sure if they are different, but maybe? Was the computer new or used? If used, did you verify capacitors weren’t leaking?
do you have original vacuum lines? If so, no cracks in them?
Is your map sensor for a 5.8? Not sure if they are different, but maybe? Was the computer new or used? If used, did you verify capacitors weren’t leaking?
do you have original vacuum lines? If so, no cracks in them?
IAC plate was originally not installed, was installed to try to mitigate the issue
Skip that idea then! You should absolutely get it to smooth out with the plate. If it won’t, something else is off for sure. What codes is the computer giving?
remove all vacuum lines from the intake, except for the line that goes to the map sensor, until you figure out your issue. You could be leaking vacuum in any number of spots that are not at the engine, best to eliminate them while you troubleshoot.
It never ran right since first start up? Is timing 12* BTDC with the spout removed?
Why? The computer will always think its set to 10*, nothing else.
Originally Posted by fishfreak101
Whats weird though and makes me thing its an issue with the speed density is that it runs flawlessly with the MAP sensor unplugged, of course at 1500 rpm though
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