1995 P30 backfiring thru intake when hot

[Cousin_Eddy1]

Hi, wondering if anyone has had a similar problem like this:

1995 southwind 33 ft P30 7.4L TBI with Banks exhaust. 73k miles. Rv gets used alot. Every 3 weeks or so.

Starts loosing power and popping after it is warmed up. Takes about 10 minutes. Wont do it when cold, happens mostly at partial throttle when cruising about 60. Also happens after starting out from a stop. Doesn't seem do it under WOT .

Facts:

1. I removed the banks cold air snorkel, problem was much worse. Points to a lean condition.
2. Idle is perfect. No signs of vacuum leaks.
3. Replaced O2 sensor
4. Replaced MAP sensor
5. Replaced fuel filter.

So, it seems to me that this starts to happen when the computer switches over to "closed loop". That means its using the O2 sensor for feedback. But i replaced that. Does anyone know what else the computer uses to control fuel rate when in closed loop? Or, any other suggestions?

Thanks!

[Cousin_Eddy1]

And Bad spark plug wire

[2bzy2c]

cncsparky wrote:

2bzy2c wrote:

cncsparky wrote:

2bzy2c wrote:
Backfiring is NOT fuel related.
There is no way fuel would cause a backfire on its own.

NOPE.

Just do a Google search "lean backfire"

Watch and learn

Thats a nice elementary explanation of some causes of backfire (as the presenter states).

However, YOU said that fuel will not cause a backfire. Wrong.

Nooo, I said "fuel will not cause a backfire ON ITS OWN!"
Be careful when you quote someone.

[2bzy2c]

I know where this is going. Not good.

The cases above while can cause backfire, are rare. I said "in 95% of cases"
What is written above are rare cases and not commonly experienced. Any time a car came in with a back fire problem, we always found the problem to be related to ignition.

Fuel on its own cannot cause a backfire, there has to be an ignition source. Most cases that ignition source is a poorly timed spark. Yes, funky valves, cam skipping a tooth, can all cause backfire, but really, how often do you see bad valves and cam timing off? Not very often.

So go ahead and dig up all the cases you want. Yes they "can" cause backfire. As a mechanic, I always go for the most common causes, then if no success, look to the unlikely.

So to summarize, I said in the middle of this thread, what to look for after the OP replaced a bunch of fuel related parts. I suggested to look at ignition as the source of the problem. The OP discovered bad ignition wire. So what more to you want?

I give up. Too many judges without a jury.

[cncsparky]

2bzy2c wrote:

cncsparky wrote:

2bzy2c wrote:
Backfiring is NOT fuel related.
There is no way fuel would cause a backfire on its own.

NOPE.

Just do a Google search "lean backfire"

Watch and learn

Thats a nice elementary explanation of some causes of backfire (as the presenter states).

However, YOU said that fuel will not cause a backfire. Wrong.

Many sites out there for you to 'read and learn' 😉

Let me help you.

http://www.2carpros.com/articles/what-is-an-engine-backfire

http://answers.edmunds.com/question-What-causes-a-backfire-166518.aspx

Most of the following came from fordfestive.com - Possible casue of Backfiring

Backfiring can be casued by the following:

Internal carburetor problems (on initial WOT acceleration, this can be caused by a defective accelerator pump).
Weak or broken valve spring (Usually severe. When it is exhaust, backfire will occur in the intake)
Low compression
Cam to crank timing off (rare, usually result in hard/no start condition)
Burnt/damaged valves or seats (sever usually)
Worn cam lobes (lack of power, can be severe backfire, often through carb)
One or more valves adjusted too tight (heard through carb then intake is usually too tight, heard through exhaust then exhaust is too tight)
Preignition (caused by deposits or lean engine conditions)
Water in fuel

Need more?

[2bzy2c]

352 wrote:

2bzy2c wrote:
Backfiring is NOT fuel related. In 95% of cases it is an ignition problem. That vintage engine, I would look to the spark plug wires, then the distributor cap.
Keep looking at all IGNITION possibilities. There is no way fuel would cause a backfire on its own. An poorly timed spark (ignition) is causing your problem.

Are you really kidding me ??????????????????????

No!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[2bzy2c]

cncsparky wrote:

2bzy2c wrote:
Backfiring is NOT fuel related.
There is no way fuel would cause a backfire on its own.

NOPE.

Just do a Google search "lean backfire"

Watch and learn

[352]

2bzy2c wrote:
Backfiring is NOT fuel related. In 95% of cases it is an ignition problem. That vintage engine, I would look to the spark plug wires, then the distributor cap.
Keep looking at all IGNITION possibilities. There is no way fuel would cause a backfire on its own. An poorly timed spark (ignition) is causing your problem.

Are you really kidding me ??????????????????????

[turbojimmy]

gswcgi wrote:
Had the same problem with a 1985 P30, 454. The heat from the manifold fried the spark plug wires and rotor about very 6,000 miles. Constantly replaced them to solve the problem. Also had massive fuel lock issues whenever the engine got hot. Had to add an additional fuel pump right in front of the fuel tank. Finally got tired of all of the P30 issues I got rid of the nightmare!

Sorry you had trouble with your P30. Suggesting the OP sell it for something else as the result of a plug wire failure probably isn't a great help, however.

I drove my new-to-me '84 P30 all summer without any vapor lock issues. I was worried about it based on everything I've read but my fears were unfounded. I can see a spot on my frame rail back by the tank where someone had installed an external pump at some point, but it must have failed and had been removed. Mine doesn't have cab A/C which can really add to under hood heat.

With the heat shields in place an proper wire routing, there shouldn't be any trouble with the wires - at least not in 6,000 miles. People talk about how close the wires and brake lines are to the manifolds, but they're no worse than the 2500 chassis trucks of the day which were very reliable.

[Cousin_Eddy1]

I was really thrown off by how it ran perfect while it was cold, then started acting up when hot. Bad spark plug wire? Didnt seem logical. This RV does have banks exhaust headers, thats gotta be hard on spark plug wires.

[gswcgi]

Had the same problem with a 1985 P30, 454. The heat from the manifold fried the spark plug wires and rotor about very 6,000 miles. Constantly replaced them to solve the problem. Also had massive fuel lock issues whenever the engine got hot. Had to add an additional fuel pump right in front of the fuel tank. Finally got tired of all of the P30 issues I got rid of the nightmare!

[turbojimmy]

Cousin_Eddy wrote:
In this case it was a plug wire with high impedence, the energy would take the path of least resistance through another wire, firing the next cylinder while the intake valve was still open.

Note to self: install the new plug wires that have been sitting in a box, on the shelf in the garage for months.

[tommykelly]

you did not mention any "ticking" noise...which is good
If you do you would want to pull your valve covers and check for rocker arms that are not moving their full travel.

comparing the rocker movement while the engine is idling one is not moving back and forth as the others...that would indicate a worn camshaft lobe.."but" you would hear a lifter clicking at the same time.

but when there is a problem...Ignition first, fuel delivery second and mechanical last is usually the best way to troubleshoot.

let us know what you find out..

[2bzy2c]

Cousin_Eddy wrote:
In this case it was a plug wire with high impedence, the energy would take the path of least resistance through another wire, firing the next cylinder while the intake valve was still open.

I rest my case.

[cncsparky]

2bzy2c wrote:
Backfiring is NOT fuel related.
There is no way fuel would cause a backfire on its own.

NOPE.

Just do a Google search "lean backfire"