Why Is Suburban "Surging" At Cruising Speeds?

fairly new to me 1987 suburban with a 7.4 and 3 speed auto. odomometer shows 48,000 (i honestly don't think it has rolled over).

mods made : throttle body spacer, jet perf. chip (stage 1), and cat. converter removal

recent repairs/attempted fixes: new fuel filter, new tranny filter and fluid, new cap,rotor,plugs and wires, tried a new egr valve(no difference), new egr solenoid, new O2 sensor

have checked for vaccum and intake leaks, none found

have bought but have not yet tried a new fuel pressureregulator. have not checked actual fuel pressure.

hard accelleration seems to be fine.

i might notice a slight "miss" at idle ??

any ideas???

 

check the fuel pressure. and may be a bad intake gasket regardless if you found a leak or not, sometimes doing a carburetor spray test around the intake can find it, sometimes it wont.

 

Try to disable the egr vavle by plugging the vacuum hose, then test drive it.
You can check the tps sensor for smooth operation.

 

tps sensor seems ok

what would disabling the EGR valve tell me? what should i expect to see when i do so?

 

The egr isn't an essential part to run, its not like a map or tps sensor.
The worst that can happen is alittle spark knock.
I hoping the surging will go away when you disable the egr.
How did you test the tps?
I wonder if the thermac (hot air to air cleaner) system is working right, if its getting hot air through that tube, it could surge.

 

i had already tried, and did so again on your advice, disconnect the egr. i do notice a little improvement in that it doesn't surge "as much" but it is still noticeable.

again i would ask, what does that tell me? for the record, i did try a new egr with no differnce.

i did also spray all around the intake with starting fluid to see if there was any change in rpm...no effect

i'll add that my timing is just about spot on at the mfr. spec of 4* (i think) i'll have to verify the indicator marks

 

oh and, i tested the tps by disconnecting the plug, put a digital ohmeter across a-b and then b-c and noted a consistent climb in value on 1 combo and consistent fall in the value on the other

hope i said that right so it makes sense

 

could the fact that i have eliminated the cat converter play into this? does the system need to see some "back-pressure" at some point that may no longer be there?

 

no cat will not affect it. that is not a problem.

 

Some aftermarket egr valves flow too much, they sometimes come w/ these inserts that you have to tap into the orfice, they restrict the flow alittle.
Did you check that air cleaner valve, thermac?

 

ok, so does the fact that i get a slight improvement when i disconnect the egr tell me anything?

as i said earlier i did indeed try a completely different/new egr with no difference/change.

yes i checked the air cleaner valve. mostly closed when cold and opens up as it gets warm.

could the map sensor be a potential problem?

 

I'd flip flop the air cleaner lid & see how it runs.

 

cant flip the lid on stock l03 air cleaners.

 

i can flip mine and have tried that....no improvement or change

again i'd ask about map sensor ??

what about the IAC ??

would the addition of the throttle body spacer and the slight change in linkages be something to look at? everything seems to still move freely and function as it should.

what is the "kick down" linkage? could that be an issue?

 

The map sensor should be checked w/ a scanner, but you can look at the hose to make sure its not cracked, add a tee & check vacuum, it should be 18" at idle & have a smooth needle sweep.
IAC is for idle only, your complaint is surging at cruise.
I don't know what tranny you have, possibly a th400, those do not have a kick down linkage, it might have a locking torque converter. Drive it till it surges, then lightly press the brake pedal, this should unlock the converter. See if the surge goes away.
Can you take a picture of the engine w/out the air cleaner?

 

i did try lightly pushing the brake while surging at cruise, it made no difference.

i'll take a picture in daylight 2moro.

i need to install the vacuum and fuel pressure guages i guess