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Misfire after idling

Sketchysedan

New member
Joined
Feb 20, 2019
I recently purchased an 87’ 245, which runs pretty good except for after I leave it at idle. In normal driving, it has a slight hesitation under light throttle, but otherwise runs great. However, if I warm it up and then leave it at idle for 15 minutes or more and go to take off, it develops an aggressive misfire. It is not noticeable at idle, and usually will rev freely in neutral, but barely stays running with any load. It feels like ignition timing is way off, sometimes it backfires too. If I get the car to move after this happens, and apply too much throttle it will try to stall out. Shutting the car off and turning it back on does nothing, however letting the car cool off to ambient temperature and restarting fixes the issue unless I let it sit at idle. What is most confusing to me is that I can take the car on long road trips without issue, but not let it idle. So far I’ve replaced the cap, rotor, plugs and wires. Any help in troubleshooting would be appreciated
 
An exhaust report is a back fire

An Intake backfire is a sneeze

They are caused by different problems

Fuel pressure test, engine off, engine running, under load, hold with key off?

Another test Id recommend is a injector leak test. Although you do not mention any starting issues.
 
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Yeah no starting issues. I had assumed it was a timing/spark issue originally but lean mixture would also do it. What is the best way to test fuel pressure on these cars with no valve on the fuel rail?
 
What is the best way to test fuel pressure on these cars with no valve on the fuel rail?

LH 2.2...remove return line to fuel tank at FPR...insert a hose into FPR, and insert this hose into a clear container, like a glass Coke bottle. With two people, hot wire fuel pumps at fuse panel, while other person watches Coke bottle. Got fuel going into Coke bottle, then rail pressure is most likely good.

Do this outdoors..not in a garage...

Your 15 minute idle issue suggests fuel was vaporizing...due to engine's heat heating fuel...so tank's fuel was getting warmed up...with intank pump working, vapor lock issue goes away.
 
Just did that test. I get fuel out of pressure regulator, and when jumping each side of fuse 4 I hear both pumps coming on.
 
I get fuel out of pressure regulator...idle for 15 minutes

What is daytime temps in your area? 15 minute run suggests fuel vaporizing. Winter blended fuel vaporizes sooner than summer grade fuel

So, at intake manifold, cap off vacuum line going to brake booster...I assume that tube from AMM (MAF) to throttle body is without leaks.

Lean mixture is the issue...with starting ether, one could spray around injectors, intake gasket with engine at idle, and away from a garage.
 
Average is about 45 Fahrenheit, so it would seem very weird for fuel to vaporize. I have checked for vacuum leaks visually and have not been able to find any but will spray carb cleaner if I have some on hand.
 
it would seem very weird for fuel to vaporize.

Rail pressure is unknown, but FPR is returning fuel to fuel-tank; returnless fuel pump systems (newer vehicles) reduce gasoline's vapor pressure in tank since fuel is not being returned from fuel rail, which picks up engine heat.

I attempt to keep my vehicles' fuel tanks topped-off...but very seldom do I idle a vehicle for 15 minutes, except in very cold weather. Plus, my stopped time is peanuts...rural driving.

Lower fuel rail pressure is possible, but if a timing light could be used at idle, with vacuum line to Chrylser ignition box removed and plugged, timing could be checked. In this thread [see post #28], if the crank is lined up to 0? mark, the distributor's rotor should be as shown, or 180? off. If 180? off, then rotate crank one full turn...on distributor's housing, there is a scribed mark which rotor should be pointing at. Is this what you see? Note - If sparks are removed, crank can be turned by hand...maybe using belts.

Does end of tailpipe have a brownish or blackish look?
 
Well looks like I found my issue. The short rubber tube between the pcv box and the flame trap had cracks in it that were not visible from the top and did no leak at idle, but when under load there was enough vacuum to pull the cracks open I guess. Either way, throttle response and idle is much better now and I have not had the issue again.
 
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