The 5.0 liter HO in my 1987 Mustang has been backfiring through the intake more and more in recent years. Sometimes it happens while under load when cold, but other times it's less predictable, like just firing up when starting, or off/on throttle when changing gears.

The engine has just over 150k miles on it, is stock with these exceptions:

- Recently added 1.7:1 roller rockers that replaced the stock 1.6:1 types (didn't change backfiring tendencies)
- 70mm throttle body
- 1.5" tubular shorty headers.

What I've done so far:

- basic tune up with new cap/rotor/plugs/wires and setting timing
- Replaced HEGO sensors
- scrubbed out build-up in intake tracts
- back-flushed each fuel injector, soaked nozzles in carb cleaner
- checked the ECT sensor with ohmmeter cold and hot (measured OK)
- ran EEC diagnostics (EVP sensor error, which it's had forever)

The only other cause I can think of for backfiring through the intake is maybe the ignition timing is unstable, like maybe the distributor shaft is worn. But that does not seem obvious to me.

What are some other causes for this?

 

Weak valve springs.
A broken or bent intake valve.

 

Thanks Conan

Weak valve springs are likely, given the mileage on the engine, and I do rev it hard some times. Of course, that might lead to bent valves as well, but I would expect that to cause problems all the time.

Checking this out would be difficult.

 

No.. do a compression and leakdown test on the motor.

 

I've been working on a lot of fords recently and of 4 of them 3 have had cut vac lines on the back of the intake and other places. Seems like they just wear and compress under vac and in time with no flow they rip. Double check that. It's free or cheap.

 

Thanks orng1 for that tip; the symptoms do seem to indicate a temporary lean condition.

When I cleaned out the fuel injectors, I had the whole upper intake off (and replaced the leaky valve cover gaskets at the same time). Putting it back together, I made sure all the vacuum lines were intact. I did break the plastic line that went from the tree to the AC system, but since I caught it, I replaced it with a rubber line. But just to see what happens with that small leak, I left it off and took it for a test drive. It definitely backfired a lot more, especially under load. So back it goes, and the engine seems to perform normally, with just that single burp every once in a while under load. The repairs I did didn't seem to change that.

One other thing I found when I took off the fuel rail was some damage in the rubber cover over the cross-over line that connected between the two sides of the fuel rail. It was pushed tightly against the back of the lower intake manifold, and vibrations started to rub through the outer rubber cover that was protecting the plastic fuel line. I guess it did its job, but I put a layer of heater hose over that for more protection. Had I not pulled off the fuel rail to do this service, I would not have seem the damage, and eventually the manifold would have worn through the outer cover, then through the fuel line, then...

So I finally did a compression test today, and all but 2 cylinders read about 179 psi; C2 showed 175 and C8 showed 173, as close as my eyeballs can tell from the coarse markings on the gauge. I don't have a leak-down tester, so I couldn't do that test. Could those 2 cylinders with the lower reading be leaking? Should I pursue them with a leak-down test?

 

No, numbers that close are good. If 1 read 110 or a noticeable difference then yes. Less than 5 percent is fine.

 

Thanks again orng1.

So it's sounding more like a temporary lean condition. Maybe I need to monitor the fuel pressure during heavy loads. Unfortunately my pressure gauge just screws onto the Schrader valve on the fuel rail, and in-cockpit pressure gauges are tricky to install. Or I can just slowly crank up the pressure regulator and see if that helps.

 

Could be a streched timing chain too.

 

Thanks Tenholes, the engine is 23 years old with 150k miles on it, so it's very likely the timing chain is worn. It came from Ford with a double-roller timing chain, and I heard that was supposed to last longer than the link-belt type chain. Although, I had to replace the front cover gasket a couple years ago, and when I checked the chain slop, it was within the specs shown in the Ford service manual (forgot what that was).

 

I vote for Timing chain!

 

Make sure you have adequate vacuum. Do you have a erratic idle? The most common problem with our fuel injected fords are the idle air bypass valve, maybe its sticking open? On my Mustang I pull the vacuum line to the fuel pressure regulator and plug the line. That will give you instant throttle response and improve your 60ft time at the track. It will at least richen you up under low rpm part throttle load and give you more information to work with.

Where do you set your timing at?

That is excellent compression, some new motors don't have that much.

 

Hmmm, I'm going to try turning the engine backwards while looking at the distributor to see how much play there is in the timing chain.

The idle is actually pretty solid, around 700 rpm. The IAC seems to be working pretty well; pulling its plug causes the idle to drop. The timing is steady at 10 btdc. But I wonder if it's possible to detect a loose chain by blipping the engine while watching for timing flutter?

I've cranked up the fuel pressure regulator by a couple PSI to see how the engine responds. Unfortunately it's been raining pretty hard the last couple of days, and it's difficult to do hard acceleration tests with a Mustang in the wet. But I'll keep an my ears open for little pops during the next drives.

 

Pulling the vacuum line to the regulator will give over 10psi. For drag racing its a big improvement. But trying to think of your symptom I can't help but think of a neighbor's pickup that would buck under low rpm part throttle load and the cause was his not very old spark plug wires. Is it a studder or a pop?

 

It's definitely a pop out the intake, like a sudden lean condition. But it's so irregular that it's hard to predict when it would do it, and the only consistency is it only does it when the throttle was being opened for applying a load to the engine. I've also checked the TPS, which showed smooth operations. Of course, that does not guarantee that there's nothing wrong with it, so I also replaced it, which did not help either.

I didn't replace the plug wires yet, but I guess I can try that next. It's hard to tell if each fix I tried actually fixed the problem, until the problem recurs. Then the answer is, no.