87 f150 302 backfire? out of intake



Could be a lot of things. But my guess is a problem with the ignition system. Ignition module isn’t consistently putting out spark, or ignition timing sensor causing the sparks to occur at the wrong engine phase… Your f150 probably uses some combo of crank/cam /dist shaft sensor for ignition timing. As mentioned above, start w/the diagnostic codes.


Well I finally did that compression test. I onlky tested 1 through 4 and here are the results
initial compression/max compression
1 100 / 160
2 75 / 135
3 100 / 150
4 100 / 160

Not sure if this points to a jumped timing chain but I sure hope so. If need be I can test the 4 on the other side as well

I should add the engine was cold.



The numbers point to a worn engine and there is too much of a discrepancy between 2 and the other 3. It will run, but…

Granted, a jumped chain can affect compression pressures but my gut feeling is a worn engine. You should really check the other 4 cylinders and should also do a wet test to see how much of a jump there will be due to rings.


Maybe I missed something but please define initial compression/max compression.


Initial was the first turn over max was what it went to after a few cranks.


If the second number is the compression after it rotates 10 or 12 revolutions (5 or 6 blips on the meter), for a cold engine those numbers don’t look too bad for an 87. It won’t run like new, but it should still run reasonably well. I don’t think compression explains your symptoms. Assuming the other 4 cylinders test within the same range I mean. Compression tests are usually done with a warm engine btw. With the throttle held wide open. The fuel and spark are disabled too.


Those compression numbers don’t indicate jumped time. Possibly a broken valve spring is causing the backfire.


I will check the other side. A broken valve spring should show as very bad compression on the one cylinder? Thanks again


A broken valve spring should not lower the compression although it could certainly cause a backfire as Rod Knox mentioned.

A weak valve spring could do the same thing. Either one will also cause a rough idle and can show up on a vacuum gauge.

I’m still wondering if the TFI module is acting up (very common with these units) and it’s not producting a SPOUT signal. Lack of a SPOUT signal can cause the ignition timing to refuse to advance and that can cause a backfire or hiccup.
Lack of a SPOUT signal can cause roughly a 10 degree retardation in idle timing if I remember correctly.

Lack of a SPOUT should also easily show up in the DTCs.


A broken valve spring can most certainly cause low compression.

If the valve spring can’t pull the valve completely closed, the compression will be low for that cylinder.

To check for a problem with the valve springs in that cylinder, do a leak-down test.

A leak-down test is where the piston for that cylinder is brought to TDC. Then compressed air is introduced into that cylinder thru the spark plug hole.

If you hear air escape out the tail pipe, there’s a problem with the exhaust valve for that cylinder. If you hear air escape out the intake, there’s a problem with the intake valve for that cylinder.



How would I test the spout signal? I have a multi meter. Or would this be a good time to just go buy a new tfi module plug it in and see if it fixes the problem.


The spout signal is disabled by removing the spout connector to set the base ignition timing.

If there were no spout signal, the engine wouldn’t run.



I’m not sure I understand. The spout signal can be disconnected to set ignition timing but the engine won’t run without it?




Timing check would be a good start. Also a bad distributor cap and rotor with a track to a cylinder is an option, assuming you have rotor and distributor cap setup, Sorry in advance nevada I do not know every car


A worn cam lobe will cause backfire but I don’t recall ever seeing a Winsor V-8 with that problem.


I have to respectfully disagree to some extent about the broken spring and compression issue. The compression stroke should close the valve enough to get a fairly accurate reading even on one wheeze. Last one I personally saw was a Chevy 350 with a broken spring on No. 6 and it still carried 150 PSI; although it did idle very rough.
Personally, I don’t think there’s a spring issue at all.

I also have to respectfully disagree that the engine will not run with the SPOUT connector disconnected.
The engine will run with the SPOUT unhooked but the timing will be shown as the base timing. Now if the PIP signal is lacking then the engine will not run.
See below for an example…

The DTCs can be pulled with a cheap code reader (15-20 bucks at AutoZone) or it can be done with a test light or analog VOM and a jumper wire. This will involve watching light pulses or VOM needle sweeps very closely. It can be a bit tricky and you really have to focus on it.


Checking the ignition timing just with a timing light fixed on the harmonic balancer is where I’d be looking on my 302 equipped Ford truck if I wanted to quickly test if the timing was more or less correct. Could test it both with the Spout connector installed, and without.


Testing the timing with the SPOUT connector still connected won’t tell you much. One would certainly not even attempt to set the timing with it connected. That would be a good way to trash the engine during some hard driving or at extended highway speeds.
George, I don’t remember the year model of your truck but I do not believe it’s a TFI model. It’s a DuraSpark and has no SPOUT connector. The TFI setup did not come along until 1985; 1984 on some LIncolns.

Another word about broken springs. Each half normally binds with the other so there is some tension on the spring and valve assembly; just not as much as with an unbroken spring.

If there were no tension at all the OP would not have been motoring around this long because in seconds the valve keepers would get chucked and the cylinder would inhale the valve followed by a catastrophic bang.


I tested the other half of the cylinders with similar results one lower but not dramatically so the rest in the same range as the others. I pulled the spout plug and checked the timing with a light and the white mark has an inch plus after the timing tooth. Which seems way wrong to me. Back to skipped chain or should I look deeper into the TFI IV stuff?