rough idle: 350 with RV cam -- ideas?
yan...@garbageacusd.edu
I replaced the stock cam with an R/V type cam (204/214 lift at .050)
in my 1990 Suburban 350 4X4 during an engine rebuild. Now I have
an irregular and rough idle - sometimes stumbling and pulsing with
vacuum varying from 10-15". No trouble codes ever show up and my
check engine light stays off. This has TBI. I have checked the vacuum
lines, replaced the EGR, and have a new oxygen sensor. It idled
smoothly before the cam change, and runs fine now at higher RPM's.
Most people tell me that this cam is mild enough to still idle fine.
Anyone have experience with this? I haven't really noticed a gain in
power (which is why I went witht this cam) and my gas mileage
dropped from 12.5 to 11.5 MPG.
Any suggestions appreciated. It is best to reply to me directly --
yandell @ acusd .edu (without the spaces) -- since my newsgroup
access is not always reliable.
Thanks in advance.
((Take out the "garbage" to reply...))
[also posted to rec.autos.4X4 ]
Robert E. Watts
Hello !
I have a LOT of experience with this cam in carbureted engines.
I can assure you that the cam profile you have described is not causing your
problems, or your cam specs are wrong.
The 204/214 cam made by just about everyone idles fine with almost factory
(17-21 inches ) vacuum levels.
I used it, or was around dozens of installs with 350 and 400 small blocks.
Still have one 400 small block around with this cam. Dead perfect smooth
idle.
I want to reiterate however, I have only experienced this cam with
carbureted engines. I can't imagine this mild cam giving the setup you
described any problems.
Now 215° or more on the intake side would definitely be a problem. Matter of
fact, I never recommend above 210° when emissions are concerned.
I would bet the problem will be found elsewhere, and I would like to hear
back from you when you find the problem.
Good luck
bobwatts
--
///////////////*****\\\\\\\\\\\\\\\
Bob Watts
Watts Carburetion Service
Whizzbang Computers
Cincinnati, Ohio
USA Earth
Since 1984
\\\\\\\\\\\\\\\*****///////////////
wats...@one.net
http://w3.one.net/~watscarb/
<yan...@garbageacusd.edu> wrote in message news:UvXT4.8$U42.109@cronkite...
Bob May
the cam is off by a tooth or you are having problems with a cylinder
or two. You might also want to check the lifters to make sure that
you don't have a collapsing one.
--
Bob May
Don't subscribe to ACCESS1 for your webserver for the low prices. The
service has
been lousy and has been poor for the last year. Bob May
Gary Glaenzer
Bob May <bob...@nethere.com> wrote in message
news:si19uh...@corp.supernews.com...
> Have you checked the compression in all the cylinders? My bet is that
> the cam is off by a tooth or you are having problems with a cylinder
> or two. You might also want to check the lifters to make sure that
> you don't have a collapsing one.
> --
Or possibly (God forbid) a lobe off that new cam..........
Roger Mauck
yan...@garbageacusd.edu wrote:
> I replaced the stock cam with an R/V type cam (204/214 lift at .050)
> in my 1990 Suburban 350 4X4 during an engine rebuild. Now I have
> an irregular and rough idle - sometimes stumbling and pulsing with
> vacuum varying from 10-15". No trouble codes ever show up and my
> check engine light stays off. This has TBI. I have checked the vacuum
> lines, replaced the EGR, and have a new oxygen sensor. It idled
> smoothly before the cam change, and runs fine now at higher RPM's.
Your learning like most people do that when you have fuel injection that
relies on a specific set of parameters you mess with one of them and things
quit working. Your bigger cam has reduced the vacuum signal to the MAP
(usually referred to as manifold absolute pressure) sensor. That means that
your fuel injection system now thinks the engine is under a load. It
therefore enriches the fuel mixture accordingly. It also compounds the
problem because the added load happens at curb idle. The throttle position
sensor is telling the computer that you are not pushing on the gas. The
computer has to pick a specific injector pulse width and timing advance
spec. based on the info it gets. The input parameters are now out of spec
so the output parameters are out of spec. All of this is true as long as
the mechanical condition is good. The O-2 sensor can correct for small
variances in the mixture caused by tune up degradation but cannot correct
for large errors in input data. Also you probably don't get codes because
the biggest error in manifold pressure is at an idle. Above idle the
manifold pressure (vacuum) is probably within the range allowed in the
factory programming of the prom. There are companies that can burn you a
prom chip to match your vehicles altered specifications. I would just put
the stock cam back in it. Also don't overlook other things that can cause
erratic idle and reduced mileage. Vacuum leaks at the intake, leaking fuel
injector/s, faulty ignition components and improper fuel pressure (too
high). It is likely the camshaft is your problem. Valve timing specs on
mass produced camshafts should be taken with a grain of salt. The timing
specs on your cam could be off as much as eight to ten degrees. With a
variation between lobes of that much also. Food for thought. Hope it helps.
Roger
Jack
timed correctly. A "RV" type cam should produce more vacuum than 10 to 15"
at idle. The purpose of a cam of this type is to increase low end torque.
The duration and overlap concerns (low vacuum at idle) of a "big" cam that
would "fool" the MAP sensor should not be the case here. More food for
thought. Also do you not mean 204/214 duration? as the .050" refers to lift.
Cam manufactures use .050" because duration numbers below that amount of
lift are, for the most part, meaningless.
Roger Mauck wrote in message <39309C05...@adelphia.net>...