Misfire under load/Hot start. 1993 4.9L
#16
05-29-2017, 08:05 PM
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Losing vacuum up hill is not normal. You could hook up a gauge to the vacuum line under the dash to check it. Maybe a weak component. Maybe vacuum leak.
I don't think the water on the distributor is a common test but since you are still here it might prove that your spark plug wires are good.
I recently hooked up a vacuum gauge at the supply to the EVR and it held longer than I wanted to wait.
I don't think the water on the distributor is a common test but since you are still here it might prove that your spark plug wires are good.
I recently hooked up a vacuum gauge at the supply to the EVR and it held longer than I wanted to wait.
#17
06-06-2017, 03:47 PM
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I ordered a new vac reservoir because that is a likely source for a leak because I have already patched it once. After cleaning the IAC the truck does start easier cold, and it does seem to idle a bit smoother. I will wait to deal with that until I figure out the vacuum problem as that lets air circumvent the IAC and cause issues.
#18
06-06-2017, 08:04 PM
Looks like you are making progress and keep checking the sensors. After reading quickly through you're post I didn't notice that you tested the fuel pressure. The 4.9 runs a higher fuel pressure than most other engines to prevent vapor lock from the nearby exhaust manifold under the intake. IIRC the pressure is supposed to be about 45 to 50 psi. If you don't want to buy a fuel pressure gauge you can rent one from most auto part stores. Make sure that it has the adapter for the Ford fuel rail Shraeder. Be careful. I usually pull the fuel pump relay and start the engine to lower the pressure in the fuel rail before installing the gauge.
My 4.9s usually hold 20" of vac at idle. I have had a vac gauge on several of my vehicles including my present truck. It was really a good thing to have in the days of carbs and mechanical-vacuum ignition systems for tuning and monitoring the engines performance. This gauge has been on everything from a 454 in a dually chassis to a 2.2 K car with several small block Ford and Chevys along the way. It is the only thing I ever installed to save gas that actually worked. Also liked it when hauling a trailer.
My 4.9s usually hold 20" of vac at idle. I have had a vac gauge on several of my vehicles including my present truck. It was really a good thing to have in the days of carbs and mechanical-vacuum ignition systems for tuning and monitoring the engines performance. This gauge has been on everything from a 454 in a dually chassis to a 2.2 K car with several small block Ford and Chevys along the way. It is the only thing I ever installed to save gas that actually worked. Also liked it when hauling a trailer.
#19
06-08-2017, 04:32 PM
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Thanks for checking in, I did put a fuel pressure gauge on after I replaced the fuel pumps (I was having crossflow so they needed to be replaced anyway) I figured the pressure was the issue but after checking with the new pumps I am making 45-50psi at idle and it jumps to 50-55ish if the vac line on the FPR is removed.
I did a fuel pressure leakdown and got:
40psi 1min after shutdown
35psi 2min after shutdown
30psi 3min
28psi 4min
This was after driving it home with the misfire going continuous for the few miles, and then squirting water on the intake manifold and fuel rail area while it was running with the missfire. There was no improvement. However after restarting following the fuel pressure leakdown it did not have the missfire.
This leads me to believe it is not an injector or fuel rail problem as that would have cleared up when I poured the water on it while the rail was pressurized causing the fuel to condense back to liquid.
I will also try capping the vac line that feeds the HVAC system tomorrow to try and isolate my leak to either HVAC or Engine Bay lines
I did a fuel pressure leakdown and got:
40psi 1min after shutdown
35psi 2min after shutdown
30psi 3min
28psi 4min
This was after driving it home with the misfire going continuous for the few miles, and then squirting water on the intake manifold and fuel rail area while it was running with the missfire. There was no improvement. However after restarting following the fuel pressure leakdown it did not have the missfire.
This leads me to believe it is not an injector or fuel rail problem as that would have cleared up when I poured the water on it while the rail was pressurized causing the fuel to condense back to liquid.
I will also try capping the vac line that feeds the HVAC system tomorrow to try and isolate my leak to either HVAC or Engine Bay lines
#20
06-09-2017, 06:42 PM
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#21
06-13-2017, 09:47 PM
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That plug with the little white "*****" or specks might be seeing some preignition.
The IAC has caused me idle speed issues but I don't remember one ever causing a problem with missing.
https://www.ford-trucks.com/forums/1...o-restart.html
Did you read this other 4.9 thread?
The IAC has caused me idle speed issues but I don't remember one ever causing a problem with missing.
https://www.ford-trucks.com/forums/1...o-restart.html
Did you read this other 4.9 thread?
#22
06-22-2017, 08:53 PM
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Alright, so the issue is definitely vacuum leaks causing it to go lean.
Replaced the Vac reservoir for the emissions stuff to minimal change. Then when I was driving cross country to Ohio, the truck started missfiring and sputtering really bad when the temps got over 110° in the California Central Valley. The truck stranded me in Bakersfield and I had to get a tow back to SLO. I will be flying to Ohio for my summer internship and dealing with this truck in the fall.
In Bakersfield I spent a few hours replacing the MAP sensor (tuned out to be a leaky hard line to it. And capped off the HVAC lines. This didn't help in the hot weather, but it did bring the level of leakage back under control for cooler weather, now the IAC does actually change the idle. My guess is that the Intake Manifold is leaking because that is the only one that would change with heat.
This issue has been rather frustrating and given the hack job that the PO did on everything I will probably end up selling this truck after graduation and buying something different or another '92-'96 F series in better condition.
I do give credit for the truck not dying on the freeway or waiting until I was beyond the AAA towing range, but the PO just beat on this thing too hard for me to trust it. I will deal with these leaks in September.
Replaced the Vac reservoir for the emissions stuff to minimal change. Then when I was driving cross country to Ohio, the truck started missfiring and sputtering really bad when the temps got over 110° in the California Central Valley. The truck stranded me in Bakersfield and I had to get a tow back to SLO. I will be flying to Ohio for my summer internship and dealing with this truck in the fall.
In Bakersfield I spent a few hours replacing the MAP sensor (tuned out to be a leaky hard line to it. And capped off the HVAC lines. This didn't help in the hot weather, but it did bring the level of leakage back under control for cooler weather, now the IAC does actually change the idle. My guess is that the Intake Manifold is leaking because that is the only one that would change with heat.
This issue has been rather frustrating and given the hack job that the PO did on everything I will probably end up selling this truck after graduation and buying something different or another '92-'96 F series in better condition.
I do give credit for the truck not dying on the freeway or waiting until I was beyond the AAA towing range, but the PO just beat on this thing too hard for me to trust it. I will deal with these leaks in September.
#23
06-23-2017, 12:29 AM
This one has had me baffled for some time. When it is hot out and the truck is warmed up it will develop a nasty misfire (shaking, tach wiggling, low on power, sounds terrible) after being left to sit for 5-10mins and being restarted, ie stopping for gas. It goes away if I let the truck cool down and then restart. That used to only happen after long stretches of hot driving like 100+°F for an hour or more. It has steadily gotten more sensitive since last summer when I first discovered it in that it will now happen at more mild temperatures and shorter run times. It has gotten to the point where it will spontaneously start misfiring up a steep grade in 4th or 5th gear but goes away once the load is decreased by downshifting or reaching the top of the hill.
Last summer I replaced the coil, cap, rotor, plugs and wires in an effort to eliminate the problem and that they were all old components. It did not fix the issue. I knew the fuel pumps were getting old because I had the crossflow issue so I replaced both pumps as well this spring to try and eliminate the problem knowing that the I6's can have this hot misfire because of vapor lock.
Computer gives only "111" pass codes when tested with the Innova reader. Fuel pressure is good at 45-50psi running, goes to 50-55ish when FPR is unhooked from vacuum.
Any tips on what to test next?
Thanks!
Last summer I replaced the coil, cap, rotor, plugs and wires in an effort to eliminate the problem and that they were all old components. It did not fix the issue. I knew the fuel pumps were getting old because I had the crossflow issue so I replaced both pumps as well this spring to try and eliminate the problem knowing that the I6's can have this hot misfire because of vapor lock.
Computer gives only "111" pass codes when tested with the Innova reader. Fuel pressure is good at 45-50psi running, goes to 50-55ish when FPR is unhooked from vacuum.
Any tips on what to test next?
Thanks!
Hope this helps.
#24
06-23-2017, 10:35 AM
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#25
06-23-2017, 01:52 PM
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#26
06-23-2017, 03:20 PM
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If you think it is a manifold leak you could block the air at the throttle body to see if it kills the engine. Don't use your hand. Make sure you are blocking the IAC too. If it keeps running you know you have a big leak. Since you mention temperature as an issue you would probably have to do it with the engine hot. You can even spray the sealing surfaces with the intake blocked to pinpoint the leaks. This would make any leak worse and may help you pinpoint the leak.
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