Reading high fuel pressure (carbureted)

1985 Caprice 5.0L 305 Carbureted (Electronic Q-jet) A/T 700R4
111,000 miles; engine rebuilt (top only) about 8 years ago

Hi all,

My car has been sluggish/chugging at idle when under load (in drive/reverse etc.) or at light/medium throttle (particularly around 20-30mph before shifting into 3rd gear).

It sounds more like a diesel engine now and the exhaust tends to "exhale" in puffs of air rather than a bit more smoothly like normal.

I checked for spark, wires, distributor, and I replaced the cap and rotor.
I then checked fuel (and replaced the fuel filter) and I still have the same problem.

I noticed the my fuel pressure is very high for some reason and I get a reading off the scale on my pressure tester -- it's over 10 psi up to about 12psi or so. It's carbureted and I think that it's supposed to only be around 5.5 to 7psi.

By the way I also checked for a restricted CAT or misc. in the exhaust by the book and it reads normal (below 2.25psi at 2500rpm). The CAT is original AFAIK. The EGR valve seems to be working okay and I checked that as well according to the book.

I do have a small vac. leak on the carburetor because the top part is warped; however, I have had this problem for a while and it is not likely the reason -- it shouldn't be any worse.

Thanks for your time.

1985 Caprice 5.0L 305 Carbureted (Electronic Q-jet) A/T 700R4
111,000 miles; engine rebuilt (top only) about 8 years ago

Hi all,

My car has been sluggish/chugging at idle when under load (in drive/reverse etc.) or at light/medium throttle (particularly around 20-30mph before shifting into 3rd gear).

It sounds more like a diesel engine now and the exhaust tends to "exhale" in puffs of air rather than a bit more smoothly like normal.

I checked for spark, wires, distributor, and I replaced the cap and rotor.
I then checked fuel (and replaced the fuel filter) and I still have the same problem.

I noticed the my fuel pressure is very high for some reason and I get a reading off the scale on my pressure tester -- it's over 10 psi up to about 12psi or so. It's carbureted and I think that it's supposed to only be around 5.5 to 7psi.

By the way I also checked for a restricted CAT or misc. in the exhaust by the book and it reads normal (below 2.25psi at 2500rpm). The CAT is original AFAIK. The EGR valve seems to be working okay and I checked that as well according to the book.

I do have a small vac. leak on the carburetor because the top part is warped; however, I have had this problem for a while and it is not likely the reason -- it shouldn't be any worse.

Thanks for your time.


check the return line for damage. re route the return line at pump into a bucket then recheck pressure..10-12 psi too high ..should be around 6psi,,...

I'll check that out as soon as I have the time. Thanks!

I looked at the pump but I can't find a return line. I see a line from the gas tank (supply w/ small vacuum i would guess) and an output line to the carb.

I had a hard time but researched more and I'm wondering if the spring in the pump is wearing or worn out. I think this can rid the limit or max pressure the pump sends out...

I looked at the pump but I can't find a return line. I see a line from the gas tank (supply w/ small vacuum i would guess) and an output line to the carb.

I had a hard time but researched more and I'm wondering if the spring in the pump is wearing or worn out. I think this can rid the limit or max pressure the pump sends out...

your fuel tank/system has 3 lines when built. vent to cannister, return to/from pump,fuel supply to inlet of pump.


internally the pump has a regulator excess fuel goes to the tank.the suction side is the inlet of pump. then the output of pump to the carb.

pump must has 3 connections to it !

normally the pump fails when the spring on the pump breaks.

on occation however the rubber diaphram ruptures causing fuel to leak out or worse get into the engine oil.the pump does have a weep hole should this rupture.

if this is the original fuel pump go and get new pump.
my engine had a lot more power with the new pump .

Hello. I have an "84" CAPRICE with the same setup that you have. The chugging symptons you describe sound like a timing chain issue to me. If the timing chain is original that means that you have the nylon toothed cam gear. These like to start schrapneling around 100k mi. The chain might jump a tooth causing the chugging you describe. Also something else to consider is the OXYGEN SENSOR. The one on my Caprice was intermitant causing issues with the electronic Q-JET. Mine had over 310k mi. when it started acting up. They are supposed to be changed out around the 50-60k mi. mark. The symptom I had of a bad oxygen sensor was that on a light throttle pull it would act like it was starving for gas. cutting in & out. Idle was normal. Full throttle was normal.
As for the fuel pump pressure problem it sounds like the pressure relief valve in the pump is not working. As J-CAT suggested I would just change out the pump for peace of mind. You also get a warranty with new parts.

HOPE THIS HELPS.

I finally got around to changing the fuel pump today. Had no issues and it now reads at about 7psi rather than double :biggrin:

It still runs mostly the same though -- chugging and hesitating just the same. I do think I should check out the timing chain some time and replace it. I will replace the O2 sensor first. It's probably been close to 30 or 40k miles so might as well just in case... it helps to remove variables and narrow down what the issue is.

I finally got around to changing the fuel pump today. Had no issues and it now reads at about 7psi rather than double :biggrin:

It still runs mostly the same though -- chugging and hesitating just the same. I do think I should check out the timing chain some time and replace it. I will replace the O2 sensor first. It's probably been close to 30 or 40k miles so might as well just in case... it helps to remove variables and narrow down what the issue is.

the fuel pressure now normal is good. the damage operating with the higher fuel pressure may have damaged the cat converter.

remove the O2 sensor and test drive the vehicle O2 hole open.

If it runs better then the cat must of been damaged.

restricted exhaust , loss of power/engine runs hotter/harder starting.

[quote=caffeineaholic;6805301]I finally got around to changing the fuel pump today. Had no issues and it now reads at about 7psi rather than double :biggrin:

I do think I should check out the timing chain some time and replace it. [ quote]

with your mileage I doubt the timing chain is in need of replacement. inspecting this is a good idea just in case the cam chain gear is worn. metal replacements are available if necessary.

[quote=j cAT;6805359]
remove the O2 sensor and test drive the vehicle O2 hole open.
/quote]

Why would you not just disconnect the wire for the O2 sensor as opposed to removing it?
Seems like it would accomplish the same thing.. :2cents:

Removing the o2 sensor will give the exhaust a place to go if the cats are indeed clogged. The car will run better then(No exhaust restriction)

Removing the o2 sensor will give the exhaust a place to go if the cats are indeed clogged. The car will run better then(No exhaust restriction)

I wouldn't think a 3/4" diameter hole would flow much exhaust but i suppose if the cat is clogged solid it would make a difference.





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Thanks guys. I'll give it a shot. I did however test back pressure a while back -- the reading was normal. Actually it was much below what the shop manual says the max should be so I thought that was good. (says 2 and 1/2 psi max and was about 1 1/4 psi)

Nevertheless I'll try this anyway. The back pressure test was before this problem.

I wouldn't think a 3/4" diameter hole would flow much exhaust but i suppose if the cat is clogged solid it would make a difference. .

with the hole open , this should improve the engine power issues, not make it run perfectly just improve it . ya its small but it will give you the needed info if the exhaust is restricted.

I have seen muffler baffles get loose and plug the exhaust it can happen also.

. ya its small but it will give you the needed info if the exhaust is restricted.

Good little trick to know..:smokin:

with the hole open , this should improve the engine power issues, not make it run perfectly just improve it . ya its small but it will give you the needed info if the exhaust is restricted.

I have seen muffler baffles get loose and plug the exhaust it can happen also.

Unfortunately it drove in the same exact way when I took the O2 sensor out. It sounded awesome though.
Also forgot to mention I have an aftermarket muffler and the resonator was deleted.

The cat could be an issue but i'm not so sure now. Nevertheless i'll replace it soon because it's the original -- and very rusty.

Could something be up with the transmission? It only stumbles/hesitates most noticeably when it goes right into 3rd gear.

I'd go back to verifying a good ignition system and checking the carb. Before adjusting anything on the carb verify the correct TPS base setting and voltage output through the entire range of travel. Mixture control, timing advance, and TCC lockup depend upon that being correct, as well as the EVAP purge and a few other items of lesser significance. After verifying the TPS and correcting it if necessary, check the lean and rich stops and set the MC solenoid mid position.

It may also be beneficial to verify the correct TV cable adjustment if in fact the trans is actually a 700-R4.