My 78 mercury marquis is hesitating

Asked by Jan 30, 2017 at 05:30 PM

Question type: Maintenance & Repair

I got a mercury marquis 1978 cranks and idles fine when
accelerating it sounds like it has bad gas in it I run 89 fuel
and I put sea foam in it but after a while of driving it
around 70 or 80 it seems like it wants to cut off like it's not
getting gas, I'll pull to the side of road and cut it on and it
pops right back on but when I put it in gear and start to
accelerate again it acts like it's not getting fuel, can
anyone tell me what it might be, it has the 6.6 400 in it

2 Answers


Is this carbureted? Check the accelerator pump, the diaphragms get old and can't pump enough gas under acceleration. When you stomp on the gas, it feels like gas cuts off for a few ticks, then catches. If you very slowly accelerate, it seems fine. Otherwise, check the fuel pump pressure, mechanical pump should be around 8-10psi. A new fuel filter might also be in order. At 70-80MPH, heat might be another problem, check if the fuel line is too close to the exhaust, and that vapor lock isn't causing you problems. This guys video shows putting a mechanical fuel pressure gauge on his 78 Marquis.

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