1991 chevy 1500 5.7 tbi erratic idle

[tpulliam79]

spring wasnt broke its all been redone, switched ecu took care of most of the issue now my 02 isnt reading very well, but i also had a bad thermostat in there so it wasnt going closed loop, thats being fixed the tranny shift points being adjusted and a bunch of new grounds. does the o2 have a ground? also hydrocarbons are very high and I have a fairly new cat?

[NEUMANNZZ]

The o2 sensor is grounded in the ecm if its a two wire.(tan wire)

[tpulliam79]

the truck isnt here but I think only one wire

[tpulliam79]

if they didnt change the cam out of the 78's short block and put the 91's heads on for the tbi , would that affect anything? vacuum or otherwise? I also had the wrong prom in the new ecm so ill know shortly about the ecm

[tpulliam79]

i am not sure if the cam is from the 78 or the 91 would it cause any issues either way?
also would the return line on the tbi unit being kinked cause any idle or richness issues?

[tpulliam79]

well what I am being told is that the cam out of the 78 blazer was used and my mechanic is saying that at idle my vacuum is 14 lbs and it should be 18 and the cam is causing it ...or something in the shortblock. is there anything else this could be the shortblock is out of a 78 with 32000 original miles and runs super strong when not at idle, I really dont want to replace the engine as I am not rich by any means. can I custom tune or anything other than tearing the engine apart? any and all help is appreciated beyond what words can say

just talked to a chevy tech and was told the cam being from the 78 shouldnt affect anything.... now im really stumped

[tpulliam79]

well my mechanic brought my truck to the house and as he pulled in the radiator drain**** burst ( which he just put in) but says its nothing he did? anyways now im replacing the radiator and realizing I dont think I trust my mechanic anymore. he says the vacuum pressure at idle is 14 and should be 18 and says that the cause is the 78 small block"s cam being different. Chevy says that wouldn't affect it? also that it dumps fuel at idle because the vacuum pressure is off. he never did check the fuel pressure.. wow sorry to rant but I am freshout of ideas and needed to clear the head....does any of this make sense to anyone?....thanks again

sorry about the rant..... does 14 sound right for vacuum at idle? and if the cam is in some way messing with vacuum and causing too much fuel at idle could a custom tune do anything for me?

[tpulliam79]

ttt

[NEUMANNZZ]

14 is low for vacuum, any leaks. valves adjusted correctly. A stock 350 cam shouldnt cause that much of an issue unless its worn out. compression and leakdown test results?

[tpulliam79]

not sure of the compression and I was told by my mechanic that the vales arent adjustable
( I dont believe him) how would I go about compression and leak down testing myself? my EX-mechanic even told me that my truck has more power than a stock 91 tbi because of the cam differance but says thats why the vacuum is off the short block and cam have just over 30000 miles on them so they shouldnt be worn out.
I am putting in a new radiator today and was thinking of replacing the distributor as the top under the rotor is very rusty and seems to have a good amount of play, is thhis normal?
I can fix just about anything once I know whats wrong but am a complete newb when it comes to diagnosis and testing. also if I do put a new distributor in how do I go about getting it lined up right, I have no timing light but I know its timed now?

[nickmac]

On your 91 there is a wire to disconnect to set the timing. It should be behind the plastic on the passenger side of the firewall. The return line being kinked could be letting it run rich because not enough fuel is getting back to the tank.

[tpulliam79]

yeah I know about the wire but I have no timing light and wouldnt know how to use it anyways, it is only dumping fuel at idle think the kink would cause that?

also if I get a new dizzy should I get the one for the 91 heads or the 78 shortblock? the camshaft is from the 78 would the wrong dizzy cause issues?

[NEUMANNZZ]

78 had hydraulic lifters. my 66 had hydraulic lifters. you can adjust the valves. distributor drive gear should be the same. get the 91 tbi distributor,
unless your gonna switch to carb then get a HEI with vacuum advance.

[tpulliam79]

looks like I need a new mechanic.... as soon as I can afford it im going to have it emissions tested then take it to chevy that way the work I have done will at least count towards a waiver. would the top of the dizzy being rusty and having play cause issues like this?

[NEUMANNZZ]

yeah if the play is more then a 1/4 of an inch. rust is normal as long as its surface rust.

[tpulliam79]

ok so if I get a new dizzy how do I go about getting it in correctly as I have no timing light?

[NEUMANNZZ]

invest in a new/used timing light.
to install distributor, before removing old distributor mark position of rotor and housing.(dont mark the old distributor)

rotor positon can be turned by cranking engine to get it number one cylinder on distributor cap for easier reference, then just mark the housing location. photos help also.

once you marked the two positions. remove hold down bolt on clamp. pull out distributor slowly. rotor will turn while pulling it out, keep this in mind when installing new distributor to line up rotor location.

[NEUMANNZZ]

A timing light is good for checking to see if timing is advancing. setting timing. checking timing to see if the ecm retards timing due to knock. checking for erratic timing(internal mechanical issues, ie timing chain loose)

[tpulliam79]

well put a stethoscope on the dizzy and it sure doesnt sound right seems like it makes a weird sound right before the idle gets funny.. how do you use a timing light?

[FordeatinZ71]

ok, maybe i've missed this...how are you running a '78 cam in a '91 block? the '91 is a roller cam...the '78 is not...again, maybe i missed it as i just skimmed this.

are you sure it's firing on all 8? might wanna do a compression test. have you replaced the coil or ignition module yet? those can both cause weak spark and the ignition module can cause the injectors not to work right.