Strange symptoms after backfire

[365Jeep]

Yesterday, I was starting the jeep (4.0), and I let off the starter just a brief moment too early. I heard what sounded like a backfire. I immediately tried to restart, and it restarted easily. Ever since then, when I push in the clutch, it will idle while oscillating between 1750 rpms and 1000. It never drops to normal idle speed. When I got home, I popped the hood and looked for any blown vacuum lines. Where should I start? No CEL. BTW, I drove it a few times today, and it's still doing it.

 

[macleanflood]

Use seafoam to check for exhaust leaks...makes thick smoke and it's easy to see leaks...start the engine, pull the big vacuum line to the brake booster...dip it in the can and run the throttle so it doesn't die.

I'd check fuel pressure.

I bought a cheap smoke machine the other day...good way to check for vacuum leaks.

-Mac

 

[365Jeep]

Not sure how much I trust this advice, but if there's a shot AND it annoys the neighbors, so be it...

 

[365Jeep]

Really? This is the best we have. I was hoping for something more definitive.

 

[freedom_in_4low]

Really? This is the best we have. I was hoping for something more definitive.

You're coming with a pretty odd, out of left field issue that most of us have probably never experienced. I dont think any of my TJs have ever backfired. When it's not a known, common issue the best we can do is start narrowing it down with general things to check that could possibly be a contributor.

High idle like that is usually gonna be a vacuum leak. I used to find those by spraying starting fluid around any suspected leak areas and listening for the idle to change. Smoke test is gonna be better.

 

[365Jeep]

You're coming with a pretty odd, out of left field issue that most of us have probably never experienced. I dont think any of my TJs have ever backfired. When it's not a known, common issue the best we can do is start narrowing it down with general things to check that could possibly be a contributor.

High idle like that is usually gonna be a vacuum leak. I used to find those by spraying starting fluid around any suspected leak areas and listening for the idle to change. Smoke test is gonna be better.

Interesting. Thanks for the response. It wasn't a loud exhaust backfire. It sounded like it came back through the intake. I did get a CEL this morning on the way to work, so I'll check that when I get home.

 

[freedom_in_4low]

Interesting. Thanks for the response. It wasn't a loud exhaust backfire. It sounded like it came back through the intake. I did get a CEL this morning on the way to work, so I'll check that when I get home.

The code behind the cel should help.

I can imagine an intake backfire could blow off a vacuum hose. I know you said you checked but there's a lot there. The brake booster is one, then there's a vacuum reservoir and the line that goes through the firewall to actuate the HVAC doors.

I haven't quite worked out how you would have gotten an intake backfire though...unless maybe when you let off the starter the engine stopped at just the right point to fire the spark on one cylinder and push the engine backward through the compression stroke and opened the intake valve.

Do you typically experience longer cranks to get it started?

 

[365Jeep]

The code behind the cel should help.

I can imagine an intake backfire could blow off a vacuum hose. I know you said you checked but there's a lot there. The brake booster is one, then there's a vacuum reservoir and the line that goes through the firewall to actuate the HVAC doors.

I haven't quite worked out how you would have gotten an intake backfire though...unless maybe when you let off the starter the engine stopped at just the right point to fire the spark on one cylinder and push the engine backward through the compression stroke and opened the intake valve.

Do you typically experience longer cranks to get it started?

It usually starts quickly. Occasionally, vary rarely, if I let off a spilt second too soon, I've heard that issue. The code was P1494, so I went back can checked more closely. There is a short vacuum line between the throttle body and the manifold. It was completely blown off. I put it back on, and it solved the problem.

 

[DuckNut]

Following post #2 would have had you back in business 12 days ago. Post #5 would have found the issue too.

Just because it may sould corny, don't discount it.

One of the easiest ways to find a bad plug wire or electrical issue is to park in the woods in complete darkness and use a squirt bottle to mist the engine compartment. Bad wires will glow blue when wet.