I have a 78 k5 blazer with a rough idle only when in drive. It had a crate engine when I got it. Not sure what they put in it. I am assuming an aftermarket cam. It has 38,000 miles on it. It has an edelbrock carb. Vacuum seems good. It seems to be starving for fuel maybe? The fuel pressure fluctuates from the pump. When in park it idles higher, but fine. Thank you for any help. Here is a video.
CK5
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Rough idle
- Thread starter CJ2
- Start date
Okay, here the deal. That filter will never be full.
You should never run a plastic filter above an engine. It will melt and start a fire. Bad enough the Edlebrock doesn't have a metal line from pump to carb.
Rough idle, could be many things. Low fuel pressure is normally not one. Low fuel pressure will show at higher speeds under heavier loads.
The vacuum reading you you had what did it says how does needle behave?
Has any work been done recently?
Pull plugs for inspection, check distributor cap and rotor.
Carb may need adjustment. Has truck been sitting ? Old gas ?
We'll get this figured out. Please remove that plastic filter at earliest opportunity
You should never run a plastic filter above an engine. It will melt and start a fire. Bad enough the Edlebrock doesn't have a metal line from pump to carb.
Rough idle, could be many things. Low fuel pressure is normally not one. Low fuel pressure will show at higher speeds under heavier loads.
The vacuum reading you you had what did it says how does needle behave?
Has any work been done recently?
Pull plugs for inspection, check distributor cap and rotor.
Carb may need adjustment. Has truck been sitting ? Old gas ?
We'll get this figured out. Please remove that plastic filter at earliest opportunity
I will remove the filter and get a metal one. It has always run like this. It likes to jerk forward when sitting at a light. It drives very well, but shakes at a light. Could it just be an aggressive cam? I had someone check vacuum and they said it was good. Not sure what it was.
It is quite possible an aggressive cam is the reason.
But there are other mechanical reasons that could be. A vacuum Guage is a small investment, and can tell us a whole bunch.
But there are other mechanical reasons that could be. A vacuum Guage is a small investment, and can tell us a whole bunch.
Ok. Will get a vacuum gauge. Good to have anyway.
Common issue with stall slowing from higher than 40 mph is the egr not closing. Did it restart easy ?
45 is not a valid reading. what you want to see 16-21 in hg and a steady needle.
45 is not a valid reading. what you want to see 16-21 in hg and a steady needle.
Sorry. It is 18.
It did not restart easy. It took like 8 seconds. Usually it is 0.5 to 2 seconds.
compare the vacuum reading to chart, this will help us narrow down an area.
According to the chart the engine is normal. I have a feeling it is just the cam. The previous owner cared a lot about the sound with bigger pipes and everything.
The truck has not shut off before at a light. However, I did recently take the line off the valve and put it back on. Will it fix itself maybe? Also, does putting a fuel regulator affect anything? Thank you
The truck has not shut off before at a light. However, I did recently take the line off the valve and put it back on. Will it fix itself maybe? Also, does putting a fuel regulator affect anything? Thank you
you took the line off the EGR valve then put it back ? Since you are in Texas and I am guessing go emissions inspection, remove and plug the EGR vacuum line.
With engine running use a soft faced hammer and give the top of the EGR a nice smack. Not enough to deform it. If Idle improves the EGR was leaking. Leave the hose off, and try to duplicate the stall situation.
A fuel pressure regulator effect the pressure delivered to the carburetor. What kind a fuel pump are you running, mechanical, lower passenger side of block, or electric, in or near tank ?
With engine running use a soft faced hammer and give the top of the EGR a nice smack. Not enough to deform it. If Idle improves the EGR was leaking. Leave the hose off, and try to duplicate the stall situation.
A fuel pressure regulator effect the pressure delivered to the carburetor. What kind a fuel pump are you running, mechanical, lower passenger side of block, or electric, in or near tank ?
I took the egr hose off for a few seconds and put it back. I did not run it without one. Sorry I should have said that. Maybe it became stuck when I messed with it.
I have a mechanical fuel pump on the lower passenger side of the block. It accelerates better since I put it on. The gauge says it is at 3 at idle and 5.5 when accelerating.
I have a mechanical fuel pump on the lower passenger side of the block. It accelerates better since I put it on. The gauge says it is at 3 at idle and 5.5 when accelerating.
fuel pressure ? 3 psi at idle and 5.5 psi while accelerating ? those pressure are fine for a carb.
The egr should operate until you are at cruise, so messing with hose at idle in the driveway is fine. If the vacuum lines are routed correctly then there is a possibility the egr could open at the wrong time. Plug the vacuum line to egr and drive it some.
With idle vacuum of 18 in and a steady needle, your engine seems sound and carb air fuel is adjust close if not spot on, fuel pressure is good. Do you know what the idle rpm in drive is ?
The egr should operate until you are at cruise, so messing with hose at idle in the driveway is fine. If the vacuum lines are routed correctly then there is a possibility the egr could open at the wrong time. Plug the vacuum line to egr and drive it some.
With idle vacuum of 18 in and a steady needle, your engine seems sound and carb air fuel is adjust close if not spot on, fuel pressure is good. Do you know what the idle rpm in drive is ?
Thank you for the help. Not sure about the rpm, but I have a tachometer ordered which will be here Friday.
The rpm in park when warmed up is 1,100.
In park with a/c on it is at 1000.
In drive it is at 760.
In drive with ac it is 680.
I noticed it also idles badly when the ac is on. It idles badly when under load. I was used the tachometer on the fan for the rpm reading. Is that correct or should it be on a different belt?
In park with a/c on it is at 1000.
In drive it is at 760.
In drive with ac it is 680.
I noticed it also idles badly when the ac is on. It idles badly when under load. I was used the tachometer on the fan for the rpm reading. Is that correct or should it be on a different belt?
