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1984 F-150 351W engine

My son bought an 1984 F-150 for the 351W engine in it. It has an automatic tranny.



It runs ok but backing it up he put gas on and it moved quickly so he let off the gas and put on the brake. It died. This repeated a number of times until he used the brake like a clutch and then it moved ok.



We just tested it and it does this in reverse and in drive. This is kind of a new problem for me, giving it gas and letting off and then it conks out.



Does anyone have any idea why this happens? I’m thinking timing, or ignition advance (vacuum or centrifugal), plug gap, point gap (it may have electronic ignition, if so does a dwell meter mean anything?)



Any help would be helpful.



Thanks,



mspart

It’s not plug gap, points (don’t have), not likely timing related, and a dwell meter is irrelevant.

This truck should be carbureted and offhand it sounds like a carburetor fault or an intake manifold vacuum leak. The carb fault (dying with the foot off the pedal) could point to a problem in the idle circuit or an air bleed. Sometimes this problem can be cleaned out with a little wrestling and sometimes it takes a carburetor overhaul.

A vacuum leak can do the same thing. About all you can do there is go over every hose on it and make sure nothing has come unplugged or split open.
A brake booster can also develop a vacuum leak. This can be tested by pinching the booster hose off with a pair of Vise Grips to see if the problem goes away.
Without truck in hand it’s difficult to be precise on this problem.

Tune it up and clean the throttle and IAC.

OK4450,

Thanks. The brake booster does not work so it is probably the source of a major vacuumn leak. We’ll work on that.

Thanks for confirming the dwell meter is irrelevant. That’s what I figured but I just couldn’t remember.

I’ll report on our findings.

Thanks again,

mspart

1981-1989, The Dark Years… These vehicles used electronically controlled carburetors and a ton of other added-on stuff to try and get these 1960’s engine designs to meet the then new emissions requirements. They were problematic to say the least. Things like carburetor part numbers changed every 6 months as manufacturers scrambled to find something that worked…

Backstage, lawsuits were flying over fuel injection design patents, with Bosch reaping huge settlements, while American and Japanese companies tried design workarounds that sometimes worked and sometimes they didn’t.

If this truck is subject to emission testing, it will always be problematic. If it has an oxygen sensor and an electrical connector on the carburetor, good luck…

Is this carbureted or fuel injected?

OK4450,

We tried the brake booster vacuum. We disconnected it and I held my thumb over it. It conked out. Then we found a small vacuum line that wasn’t hooked to anything. So I plugged that and we tried it and it worked fine. I thought that was weird because it is such a small little line. But sure enough it worked. I unplugged it and it still worked, so it wasn’t that. Then I looked at the booster and the line was completely off. So the engine was sucking a lot of air through that monster vacuum line to the booster. That made it not conk out. When we plugged it back to the booster, it would conk out.

Again, the symptom is that it conks out when the accelerator is hit kind of hard and then released just as fast. The engine just dies. Well, with the huge brake booster vacuum line disconnected and sucking air for all it’s worth, it doesn’t conk. I imagine that it would not run well on the road in that condition, but it seems to be more stable at idle.

Any help would be most appreciated.

Thank you,

mspart

Thanks. The brake booster does not work so it is probably the source of a major vacuumn leak. We’ll work on that.

Thanks for confirming the dwell meter is irrelevant. That’s what I figured but I just couldn’t remember.

I’ll report on our findings.

Thanks again,

With our findings that the vacuum line made the car run more stable, I’m wondering if the reason is the idle mixture is too rich. That would explain why increased air flow through the manifold would keep the car more stable. The engine idles but it is not the best. I’m wondering if we backed the idle screw a bit if that would change this whole dynamic.

Any thoughts?

mspart

What are we dealing with??? Is it carbureted or injected??? This is when they changed over… If it has a carburetor, there will be TWO idle mixture screws and they were sealed at the factory. To access them, plugs must be drilled out and special tools used to adjust the screws. Or has the carb been replaced with an older, unsealed unit? If so, what type of carb??

No IAC on this 1984 model.

An 84 351 should be carbureted.

Caddyman,

It is carbureted. I don’t know about the carb whether it has been changed or what. I don’t have many particulars as it is new to us. I’m wondering if my theory has any validity.

mspart

I think that your theory that it is running too rich is very valid. However, it is unlikely that just an idle mixture adjustment will fix it. I suspect that you will have to tear into the carburetor.

Can you post a picture of the top of the engine with the air-cleaner removed?