Round 3 rough idle, and other stuff too..

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89 Chevy Silverado 1500 series 1/2 Ton Truck w/ a 5.7L engine & a 700r4 auto transmission...it is not a 4x4. Recent engine rebuild. (600 miles ago)

ok, when cold the engine idles rough, when warm seems to run fine although the engine temperature runs about 195-210 at hwy speeds. the cooling system is functioning properly....plugs,rotor,cap & wires are all new...as are the air and fuel filter and pcv valve. 02 sensor was also replaced when rebuilt. I recently had trouble with egr system (code 32) was a leaky solenoid and a leaky egr valve....I replaced both. no more code 32's.

no codes are being stored at all. with a chiltons manual and a multimeter I have check the following....TPS, MAP, IAC (cleaned passages and terminals as well),ignition coil...all measurements (I did not deviate from the book) were within specs, I triple checked. Valve Lash is also by the book. I also have checked for vacuum leaks and intake manifold leaks w/ a can of starting fluid, i found nothing. A visual inspection of the fuel injectors while engine running...both injectors seems to be spraying even amounts w/ no erratic streams. I also checked for exhaust leaks, nothing. I pulled the spark plugs....each one of them had light brownish markings on one side. I showed them to a friend and he stated that it appears to be running lean. When I timed it I did so at normal operating temperature in bypass mode @ 4 degrees BTDC (by the chiltons manual).

am I missing something that may cause this condition but does not cause the ECM to store a code? I feel as if I am going :screwy:

posted by  Cantesoleares

during engine rebuild did you replace the camshaft and/or timing gears
also maybe have a ignition system check done for a bad wire or spark plug(high resistance)

posted by  osborste

Yes, I replaced both.

posted by  Cantesoleares

thats good rules out worn lobes on cam and worn gears/stretched chain

have you done a compression test or have one done

posted by  osborste

reading my manual haynes chev/gmc1988-2000 model the5.0/ 5.7 timing should tdc 0 degrees for 88-95 models the 7.4L is 4 deg btdc not out by much but might make a difference if the emission sticker is still on the rad shoud check it for timing spec

posted by  osborste

No I have not....I will do a compression check in the morning. I was also thinking of testing the fuel pressure from the pump (the pump is over 200k miles old) and also running some fuel into a container to check for water or rust contamination....or would this be looking in the wrong area?

Thanx osborste, my experience w/ mechanics is limited...and mainly textbook. I have tried to repair and rebuild a worn out vehicle before (a ford probe) and failed (parts were too expensive, so there wasnt much room for error on my part, and I made mistakes)...I would like to succeed for a change...this is sort of a personal challenge for me.

posted by  Cantesoleares

ok...compression check finished.....I removed each spark-plug one by one. Compression on each cylinder was between 115-120. Is this ok?

Also, my chiltons does say 4 degrees BTDC (I rechecked)

......would the elimination of the IAC, TPS, MAP, vacuum leaks, manifold leaks, exhaust leaks, cylinder compression, valve timing, and air filter constitute moving towards the fuel delivery and ignition sections? I did not have a fuel pressure tester (it was a vacuum tester) so I did not get to test the fuel pump today.

posted by  Cantesoleares

you right i looked up the timing 4 deg btdc my book is wrong
fuel psi 9-13 psi on tbi units is this in your book
i was hoping the compression was going to be higher but at least they are close my book has a spec of 150 psi for compression(was the engine at operating temp when you did your test and did you recheck with a couple of squirts of oil in the cylinder(help for a better seal on piston/rings for checking valve seating/head gasket) as for vacuum i don't have a spec for this just a comparision of gauge readings check your book for vacuum test at the intake manifold

posted by  osborste

No I did not squirt any oil into the cylinder, the engine was not at operating temp...

posted by  Cantesoleares

today, while looking at the engine idle and hoping to have a mental breakthrough, I heard a large hissing sound. At first I thought it was a huge vacuum leak. I then took off the air cleaner to inspect it and the throttle body was opening ALOT. It was sucking in a huge amount of air. The rpm's were not changing though. I turned off the truck....and when I restarted it....it no longer happened :screwy:

posted by  Cantesoleares

so you still have rough idle?
i'm wondering if the valve adjustment might be a little tight causing the valves not to seat fully giving you the lower compression if you have access to an air compressior(need adapter for spark plug hole or a good blowgun with big rubber tip) try checking the cylinders one at a time with valves closed and piston bottom dead center if the adjustment is to tight holding valves off seat the air will leak out throttle or exhaust or both might find one or two cylinders. maybe check the cylinders you got the lower comprssion first

posted by  osborste

I readjusted the valve lash yesterday just to be sure (someone else told me the exact same thing)...heres the thing...it runs excellent when warm....only when its been sitting for over 4 hours does this happen. Idles like crap until the engine warms up then runs great...all except for the warm hwy temps

posted by  Cantesoleares

check the torque on intake retorque the bolts as the engine heats up(expands) it might be sealing a small intake leak

posted by  osborste

read through the posts running hot your plugs light brown (running lean)
need to get fuel psi checked thinking its running lean(runs hotter)not enough fuel delivery(psi regulator or bad injecters or pump ) or vacuum leak(till it warms up)

posted by  osborste

ok...manifold bolts properly torqued, fuel psi is a steady 13.....I do not know how to test the injectors and I cant find the fuel pressure regulator.

When I re-adjusted the valve lash, they were tight (not by much), they did need to be re-adjusted (the lifter raised a small amount when I backed the nut off of it) I had an experienced mechanic watch over me to ensure I was doing it correctly this time....chalk this up to my inexperiece. I will install new spark plugs in the morning and see if it still runs lean.
this did not fix the cold idle problem however. is there another way for me to check for manifold leaks other than the technique I have already used?

posted by  Cantesoleares

steady 13 psi is great the regulater is between the injecters part of throttle body at that psi you have plenty of pressure but hows the volume now. i've used a propane torch un-lit to check for leak the starting fluid is ok to use to. it's just to seal the leak for a few second to see if idle gets better
does your manual show how to remove the injecters from throttle body ? the injecters may be plugging still giving the fuel psi but not the volume/flow

posted by  osborste

yes my manual does show how to remove the injectors....what should I be looking for?

posted by  Cantesoleares

if it tells you how to test them with a multi meter or cleaning

posted by  osborste

Nothing..it only shows removal and installation.

Note: when i checked fuel pressure I used a glass jar to examine a small amount of gas that drained from the fuel line...no water or other visible contamination. It looks like its a plastic fuel cell (silly me checking for rust :laughing: )

should I buy new plugs since these were running lean...or would my hard earned cash just be wasted?

posted by  Cantesoleares

double check that you have the right plugs(heat range)if posible go with the ac delcos matched to the emission sticker on the rad support

posted by  osborste

you mentioned the tps,and others,but do you have a coolant temp sensor that is sending the wrong info to the ecm?just a thought,if you have checked
all the rest,you might have a temperature sensitive component.if you go to an electronics supplier in your area,there is a can of "freeze spray "that is about 10 $.if an electronics component or module is faulty,let the engine warm until the problem goes away,then freeze the suspected parts.the problem will re appear.i have used this product to find hundreds of bad transistors,diodes,rectifier blocks,ic chips,even cracked pc boards.

posted by  the lobster

i think lobster hit the nail on the head the temp sensor if it sending info to the ecm that the engine is warmer(you higher temp 195) than it actually is it would think it needs less fuel(your lean condition) kind of like a choke opening to soon the engine idle would be affected

there's 2 sensors one for the gauge single wire driver side exhaust manifold between the plugs and 2 wire for ecm at thermostat housing replace this one or both might fix the temp gauge to

posted by  osborste

the 2 wire temp sensor @ the thermostat is new (not because it was ever bad but because I broke it when removing the front accessory brackets) and the single wire lead for controlling the temp guage seems to work as it should, it only registers the higher temps when running @ 65 for a half hour...other than that it reads just as it should.

so, after reading back through this post and doing alot of research I think I may have an intake manifold leak that I am unable to detect. I am going to purchase new gaskets and make absolutely sure I get a great seal...If this does not work I will take the front cover off the timing gears to inspect that the timing marks are lined up correctly...If this does not reveal my problem then I will break down and take it to someone. It may be a while but I will report back as to what the problem was, thanx for all the help.

posted by  Cantesoleares

ok, decided not to remove the manifold (checked again for leaks and found nothing) or timing cover....I am just going to stop fighting it and take it to a pro.

...but before I do I wanted to say that I was thinking alot about what lobster had said....when I installed the temperature sensor by the thermostat I used silicone to ensure against leaks....could I have insulated it and caused it not to perfom properly??

posted by  Cantesoleares

thats funny i was thinking that too(did you use teflon tape or something to seal it ) the silicone could be acting as an insulator but i'm looking at the wiring diagram the throttle pos sensor shares a wire(black wire sensor return to ecm) with the coolant temp sensor i'm thinking tps or its miss adjusted this would affect the idle as the ecm would see incorrect voltage at idle

posted by  osborste

Purchased a DIGITAL multimeter to re-check the sensors once again (I was using an old one that was hard to read)....all sensors checked out fine except the MAP sensor.

Key on engine off:

Terminals A and C = 5 volts
Terminals A and B = 1.1 volt (I am 4500ft above sea level)

Engine running:

Terminals A and B = 4 volts @ idle
Terminals A and B = 3.7-4.1 volts @ WOT

I could not find any vacuum leaks......with this startling new discovery should I postpone a trip to the shop and replace the MAP?

posted by  Cantesoleares

yep the volts at idle should be .5 to 2.0 volts max

posted by  osborste

ok...swapped it out with a known working MAP....exact same readings as before, re-installed my MAP. so now we know the MAP sensor is functioning properly..

keeping in line with the votages I stated in a previous post what then could be causing the MAP to read those voltages?

posted by  Cantesoleares

you have a vacuum gauge check the manifold vacuum at the map sensor at idle

posted by  osborste

16.5 at idle and 18 when I open the throttle

posted by  Cantesoleares

had to find mine
for that low of vacuum my gauge say late ign timing 18-22 range would be normal leave the gauge on and try turning the distributor see if you can bring up the vac at idle to 18-20

posted by  osborste

no such luck

posted by  Cantesoleares

can you double check your gauge on another engine see if you get a better reading off a normal running engine

posted by  osborste

not at the moment....guage is "out of the box" new though. It seems the MAP is just reading what the vacuum guage is reading.

Low vacuum at idle (16.5psi) and a very small increase (to 18psi) when throttle is nearly wide open.

posted by  Cantesoleares

at idle that vacuum should be higher there's gotta be a vacuum leak somewhere. leave the gauge on the map hose using a pair of needle nose pinch different vacuum hose's to see if you can get the reading up don't forget the brake booster hose the diaphram could have a small leak inside the booster

posted by  osborste

alright....I went through every vacuum hose that connects to the intake....disconnecting, plugging the port on the throttle body, still no change. I also disconnected the brake booster and plugged the hose (to rule out the brake booster) and found nothing

posted by  Cantesoleares

i dont get it the only sensor you haven't checked is the iac(idle air control)valve and it controls air flow through a bypass in the throttle body for idle maybe test it or follow the instructions in the manual about the pintle extension distance while its out you can spray some throttle body cleaner in the opening after this i'm running out of idea's

posted by  osborste

How about this: Its a Chevy, period. No more needs to be said.

posted by  FordFromHell351

i'll take a chevy truck over a ford car :2cents:

posted by  osborste

The IAC test in the chiltons is a resistance test....I got 40ohms between terminal A&B and C&D and an infinite reading between B&C and A&D. I then removed it and cleaned it as well as the throttle body passage...no change.

So..all the sensors seem to be working properly including the MAP. I have a low vacuum/high manifold pressure problem. The MAP sensor (which I know to be good) is reading the same manifold pressure at idle as it is @ an open throttle. A vacuum guage confirmed these findings. I have excluded any external vacuum leaks from the throttle body or intake manifold. The ignition timing is correct, as is the valve lash.

I am hoping this condition could cause A: the fuel to be too rich at idle....which would account for the poor idle when cold...and B: a lean condition at high speeds...which may be contributing to the high temp readings.

Would I be correct in assuming that either I have A: an Intake Manifold Leak or B: Incorrect Vave Timing ( a tooth off on the timing gears)??!?

posted by  Cantesoleares

:laughing: I think the real problem here is I did all the work....the last time I ruined an automobile...it WAS a ford.

posted by  Cantesoleares

a vacuum leak should give a lean condition at idle(more air than fuel) if it seals when hot should be normal or close to it
valve timing should affect idle and compression as the valve would open/ close at the wrong time thats gotta be it you have idle problems and low compression for a rebuilt motor
at least you have ruled out all posibilities before tearing it apart to check this

posted by  osborste

reading the manual low vacuum at a steady reading
1 leaking intake manifold or throttle body
2 incorrect valve timing
:thumbs:

posted by  osborste

Could also be a burnt or poorly lapped valve or a weak valve spring. Pull off the valve cover and acually measure the valve when it is closed fully from the valve spring seat. If you get a lot of variation between the measurements from intake valve to intake valve, or exhaust valve to exhaust valve, then that is your problem. :2cents:

posted by  srober32

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