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Rough Idle help

Sgt.Waterfowl

Sgt.Waterfowl

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So my "new to me" '78 Blazer just started idling and running rough. Wouldn't idle very well and ran like it was running and sounded on 7 cylinders. Changed the ignition system including dist. cap and rotor, plugs, wires. Still ran rough. Was going to spray out the carb. and check for a leak on the egr valve. Any help with diagnosis of vacuum leak or such is appreciated.
 
Plugs looked very good. There was some oil around the threads, figured that would be expected. Also I don't believe this year has an EGR valve, or I cant find it. :(
 
Every GM V8 since 1973 had an EGR valve factory--if someone put an aftermarket intake on it it may no longer have one,or the original could have been removed and blocked off..

Easy way to check for vacuum leaks is to use a spray can of carb cleaner,put the straw in the nozzle and leave it idling ,and squirt any suspected areas you think a gasket may be failing,etc--the idle will noticeably change if there is a leak and the carb cleaner gets sucked in..

There is probably hundreds of things that can cause your rough idle--it may be a cylinder is low on compression due to a leaky or sticking valve as noted,usually you'll get a backfire from the exhaust or the carb if a valve is causing it...a compression test may narrow it down to one cylinder...I've had rocker arms fail and the engine ran rough ,but there was no tappet noises before -(bought a good car cheap because of that,owner thought the engine was on its way out)..one engine I had bent a push rod twice in a week,then the rocker arm stud pulled out of the head...

Another possibility is a cam lobe has gone flat,but that usually shows up more at high rpms than at idle..

It is easy to mix up the #5 and #7 spark plug wires,make sure you have the firing order right...also I'd only use AC spark plugs,Bosch platinum plugs are noted for not firing right in Chevy engines for some reason,I've seen engines run worse than before the "tune up" when Bosch plugs were installed,and putting the old plugs in "fixed" it!..
 
Well I sprayed the carb out with cleaner and while it was idling I sprayed around all the vacuum lines and gaskets with no change in idle. After all that is was idling higher and when I pressed the pedal down it went back to rough idle. It does sound like its "pinging" under the drive side valve cover in front two cylinders. Could this be just from the rough idle or like you say, maybe a rod or cam lobe causing this sound. And definitely no EGR on the intake manifold, would it be behind the distr. ? Gonna swap out the coil now since that is the only thing besides distributor that hasn't been changed. Thanks for all the help and info so far.
 
I doubt if its the coil,usually they either work good or don't,and weak spark will show up more under a load than just idling in park...usually if a coil or distributor cap cracks it'll skip at all speeds and mostly under a load,but you never know..

If the idle speed increased after you sprayed the carb cleaner on suspected vacuum leak areas it could indicate there was a leak and you temporarily sealed it with the fluid,and when you gave it some gas it sucked in the rest of the cleaner and let the air in again...
The base gasket on the carb is always a suspect,they can fail and not have an evident vacuum leak on the outside,but let vacuum get in a passage under the carb where none should be..

As for the noises under the valve cover,the easiest way to diagnose those is to take the valve cover off and look--if any push rods look bent,or a rocker arm does not seem to line up good with a valve,or has a lot of excess slop or lash it may have lost a cam lobe or pulled a stud out..I've had a few rocker arms break in half too,and one also had the push rod "drill" right thru the rocker arm !...(these were on high mileage engines that had did not get the oil changed often enough)..

You can idle the engine with the valve cover off a short time and watch the rockers,see if all seem to be moving the same amount..(oil will splatter everywhere,so be quick about it!)..I have put duct tape over the push rod oil holes after cleaning oil off the rocker so the tape will stay on long enough to do the test..if one seems to not have as much travel as the others,there is a problem..

Another possibility is a lifter may have "pumped up" and stuck there,and is holding a valve slightly open--stock Chevy lifter's are noted for that if you rev them up too high,I have had a few lifters push the little clip out and let the inner "piston" to be pushed too far up and get stuck there...have you done any wide open throttle runs lately ?...:thinking:..
 
No wide open throttle runs. Got this for my 16yo daughter a cpl months ago and she actually drives it pretty conservatively. I didn't know what it was sounding or running like until she said it sounded rough and kept dying when she tried to back out of a parking stall. I'm just 4 months into recovering from a mc accident which broke my pelvis and other parts so I think my limit of getting in that compartment to diagnose is at its end. Gonna have to probably bring it in and let someone more maneuverable get in there. Thanks for all the info and suggestions.
 
No problem,wish you were able to find a simple fix--it probably is not anything major hopefully...bad gas may be a possibility but usually the engine will not want to pull a load with water or dirt in the gas either,not just idle rough--could be crap in the carb's idle circuit..

Have you checked the PVC valve for good vacuum ?--if its hose collapses or plugs with carbon or the valve sticks,it can affect the idle a lot--also don't overlook the brake booster and transmission modulator for possible sources of vacuum leaks..
 
P.S.--sorry to hear about your accident,A busted pelvis sounds very painful--I wouldn't push my luck trying to do to much too soon!..
I can sympathize with you,not wanting to crawl up into the engine compartment--I can barely do it myself now,with my back always killing me --I hate having to work on a tall truck when the parts you need to reach are just behind the radio,like the distributor...

I put a vacuum pump canister on my diesel that is about in the same spot and I ached for a week..had to lay planks across the fenders and lay across them..when I had to replace the drivers side exhaust manifold and had to torch off all the bolts,then extract them,it took me 3 days of all afternoons to do it,and I think I'll never fully recover from that ordeal...it was 6 months ago and my ribs still hurt from leaning over the fender...crawling out from under the truck 100 times did me no justice either..
Unfortunately I have no other mechanics that will work on "such an old rusted p-o-s" I can trust..most shops I've asked to fix anything flatly refuse to work on it--or quote me an outrageous price,hoping I'll go away..one guy wanted $850 to put a good used oil pan on!--it might take a whole hour if you had a lift!..:screwy:..so I end up doing it myself,and wish I hadn't later..:(..
 
Appreciate the thoughts. Yeah I think Ill have to bring it in and get it diagnosed, if that's all possible without throwing parts at it. If its an easy enough fix I might try and convince myself with a few pain meds to get in there and do the labor. Anyway Ill post back when it gets fixed :confused:
 
So I cant get the blazer in until Thursday which gives me a couple days to work up the energy to get back in there. I changed the PCV and Vacuum advance on the dizzy. No EGR valve on this manifold. Timing was just a hair above 8*. Still runs rough as heck at idle and slow speeds. Higher RPMs keeps it running but no pick up and lags during acceleration. Adjusting the carb, I suspect would only increase the idling better but have no help during higher RPMs ? I guess I can start looking at the fuel sytem. :dunno:
 
otherwise sounds like a flat lobe on cam, lifter, pushrod, rocker arm type situation. easiest thing to do is borrow a compression tester and/or pull valvecover and watch the rocker arms.
 
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Sgt.Waterfowl said:
So my "new to me" '78 Blazer just started idling and running rough. Wouldn't idle very well and ran like it was running and sounded on 7 cylinders. Changed the ignition system including dist. cap and rotor, plugs, wires. Still ran rough. Was going to spray out the carb. and check for a leak on the egr valve. Any help with diagnosis of vacuum leak or such is appreciated.
Click to expand...

I broke a rocker once causing similar.

Happened to be on the side with the oil fill and could here it rattling with the oil cap off.
 
I have also had a quadrajet cause my 81 Scottsdale to run like it was missing a cylinder.
 
I've had more than one Chvy V8 with a flat cam lobe,and the compression test in all cylinders came back "good"...

Evidently at cranking speed,it doesn't take much valve lift to get enough air in the cylinder to compress--so it'll have "normal" compression ,still idle OK,it is at higher rpms you'll hear the "gargling" at the carb and possibly a spit back or backfire out of the carb-it was so common for Chevy's to eat a cam lobe I could diagnose them just by listening to the carb when I'd rev it up..
The best "test" is a visual inspection of the movement of the rockers with it running,its faster and easier than trying to measure the lift with a dial indicator...usually the dead lobe is easy to see by the lack of travel at the rocker arm..

-most of the flat cam lobes I've found were on the #1 cylinder and it was an exhaust valve that lost its lift--- because the cylinder cant "empty" and let out all the exhaust fast enough, it escapes thru the intake next time the intake valve opens..one 454 I had got so bad it backfired and set the air filter on fire one night when I revved it up to pass someone!..:eek:..
 
I've had more than one Chvy V8 with a flat cam lobe,and the compression test in all cylinders came back "good"...

Evidently at cranking speed,it doesn't take much valve lift to get enough air in the cylinder to compress--so it'll have "normal" compression ,still idle OK,it is at higher rpms you'll hear the "gargling" at the carb and possibly a spit back or backfire out of the carb-it was so common for Chevy's to eat a cam lobe I could diagnose them just by listening to the carb when I'd rev it up..
The best "test" is a visual inspection of the movement of the rockers with it running,its faster and easier than trying to measure the lift with a dial indicator...usually the dead lobe is easy to see by the lack of travel at the rocker arm..

-most of the flat cam lobes I've found were on the #1 cylinder and it was an exhaust valve that lost its lift--- because the cylinder cant "empty" and let out all the exhaust fast enough, it escapes thru the intake next time the intake valve opens..one 454 I had got so bad it backfired and set the air filter on fire one night when I revved it up to pass someone!..:eek:..
 
Literally just dealt with this last week and no lifter/rocker rattle, since when it's completely off the push rod it makes no noise ....... started running rough couldn't find the cause so pulled vc and found #6 exhaust rocker completely off
 
So the latest update. Its been at the shop for while :mad1: . Only thing they've done was adjust the valves, and rotate dist. The timing was a smidge above 8* when I brought it in and they said when they checked it was nearly 20* :confused: I was told it sounds worst like its running on 3 cylinders, which was not the case prior, maybe 7 when I first took it in. lol. Was told when they took plug wires off to attempt and isolate the cylinders it made no difference with how it ran. Gonna check compression and possibly chk if timing chain jumped. Any thoughts ????:1zhelp:
 
Sounds like you need a new shop (mechanic),if they had it this long and still dont know whats wrong with it..:dunno:

Aside from everything we've listed previously to check,I would see that those cylinders that are not making any difference with the plug wires pulled off actually are getting spark,it could be as simple as a bad distributor cap or pick up coil,or reluctor--then if they are getting spark,check compression..its pretty unusual for several cylinders to stop working suddenly--even a head gasket blown between two cylinders only lets two misfire or act dead..

Most of the time if an engine jumps time it will either not start,just throw flames out of the carb,or backfire,or it will still run but cranks over hard like the timing is way too advanced,and will run like crap..
It would have moved more than 12 degrees probably too,if it jumped..but I suppose it could have jumped a tooth..it is a lot of work to tear it down that far--and find out the chain is still OK...

I had a 351W Ford jump time out in the boonies one nigh long ago,friends and I were out joyriding and were rather buzzed on beer--we got it to run by removing all the plug wires from the cap and moved them all over one hole,and turned the distributor while another guy cranked it over--the engine started,ran like it had 3 or 4 cylinders,but we were able to limp it home..
 
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