93 Sonoma 4.3 'Z' : rough idle & bad MPG

Greetings, new member here!

I have a 93 sonoma, 4.3L v6 engine code 'Z' (TBI), 161,000 miles, 4L60E transmission. Clutch fan replaced with electric fan conversion kit. ODB I computer.

Back in sept 06, I took my truck to the dealer and they replaced a shorted EGR solenoid and replaced the EGR valve. Then they welded a crack in my 'Y' pipe. So far so good, so I thought.

A few weeks later, my truck wouldn't heat up. Had the thermostat replaced by a different shop. Then my truck started running badly at idle and lost 1/3 of my MPG. Was getting 18-20 MPG, but now about 11-13 MPG. Same shop then ordered 'Flow Tested' injectors from Indianapolis & installed the same. Still bad idle and running badly at idle. The shop owner told me that none of the 4.3's run smoothly. :nono: I disagree. I keep a DOME BOOK on the truck for the past 3 years and the truck ran quite nicely until recently.

Next I went to CarX and got 2 exhaust leaks fixed (one started leaking again where the convertor connects to the pipe going to the muffler) Exhaust still seems a bit LOUD even though it had been replaced a couple of years ago from the convertor on back.

Took my truck to a different shop. They said my aftermarket replacement electric fan relay temperature sensor should be moved from the radiator inlet to the outlet. So, I moved it and replaced the thermostat again. Thermostat opened about 200F then dropped to about 150F and wouldn't go up again was the reason for changing it the 2nd time. I told the guy that I was getting about 20MPG and he called me a liar! :disappoin (I have records to prove it).

Next, I went to a junk yard and bought the whole top 1/2 of a TBI (same type and year) so I could be sure that the fuel pump regulator and injectors could be ruled out. This helped slightly, but to no avail.

Symptoms:

For the first 2 minutes, cold start, the truck runs OK, but not like it used to. About the 2 minute mark, the engine starts missing badly for about 10 minutes. Then smooths out a little. Another 5 minutes later when the temp gauge indicates about 190-200F, and you are idling, it runs really rough, like 1/2 the engine is missing. The truck runs like a 'scalded dog' when driving it.

Compression test revealed 175 lowest, 195 highest, and low 180's for the rest of the cylinders. Highest and lowest are on opposite corners of the engine. I tried checking the vacuum. If I checked it correctly, the gauge said 14 - 16" vacuum at idle. What is the normal vacuum anyway?

Gas mileage went from 18-20 down to 11-13. :shakehead


I have new Bosch spark plug wires, a/c delco R45's for new plugs (to ensure I get fire), new distributor cap & rotor button. One shop told me that my FUEL PSI was around 11 or 12 and the normal was 9 to 16.

I'm tired of getting ripped off by paying for diagnostic's and not getting a proper diagnosis. :runaround: From what I can see, the repair shops where I live are too lazy to 'do it right' and check things to old fashioned way - with a meter or SUN Machine. All they want to do is hook up a hand scanner and replace parts. :banghead:

It's getting too expensive to just keep taking it to the shop and having them guess at the problem while charging me $75/hour + inflated parts markup.:crying:

Can anyone tell me what to check, how to check it, and what values should be the norm for any suggested diagnostics??

Checking the voltage on my TPS won't help me any - I need resistance values for TPS and Coolant Temperature Sensor in addition to any other tips you can give.

Haynes Manual doesn't give enough information other than to casually check things and how to replace them.


Feel free to email me if necessary.

Thank you!

Welcome to AF.
Your vacuum is low.
Things to check is engine base timing and egr valve stuck open and any large vacuum leaks.
Is the engine reaching and holding 195 degree operating tempt>?
Also check for plugged exhaust system by.
Checking vacuum at idle and at 2000 rpm and post back reading.
Any check engine lights?
You can check for codes by jumping A and B pins on diagnostic connector.
If any codes post back code.

I was going to check the timing today, but had some work to do. Unfortunately, the weather here is going to turn really cold and I won't be able to check anything.

I don't have any check engine lights.

Also, could the dealer put on a defective EGR valve or gasket??

Could a clogged Catalytic Converter cause the same symptoms? Or partially clogged?

:dunno:

Could a clogged Catalytic Converter cause the same symptoms? Or partially clogged?

Yes and no.

A vacuum test at idel and at 2000 rpm will tell you if a plugged cat or exhaust system.

Remove and plug egr valve ports and see if vacuum comes up.
If so your egr valve is leaking or sticking.

Even if you do not have a check engine light pull codes and if any codes post back.
To check timing you will need to put it in base time check by disconnecting the bypas wire.
And let us know if engine is holding 195 degree runing tempt>
Good Luck and let us know how it goes.
MT

As soon as I can get a chance, I'll do as you recommend.
Temp gauge fluctuates between 190 and 200 from what I can tell after about 15 minutes or so depending on outside temperature.

What should I plug the EGR ports with?


Where would the best place be to test for vacuum? PCV line?

I'll pull the codes tomorrow (top 2 of the test port right?)

OOPS! Where's the wire I need to disconnect for the timing advance?
Note: One shop told me that my harmonic balancer 'slipped' a little.

As soon as I can get a chance, I'll do as you recommend.
Temp gauge fluctuates between 190 and 200 from what I can tell after about 15 minutes or so depending on outside temperature.

What should I plug the EGR ports with?
Answer
Remove egr valve.
I use masking/duck/black tape on the suction port.
Let the exhaust shoot out.


Where would the best place be to test for vacuum? PCV line?
Answer
Find main engine vacuum port.

I'll pull the codes tomorrow (top 2 of the test port right?)
Answer top right A & B

http://www.troublecodes.net/GMhttp://www.troublecodes.net/GM/

OOPS! Where's the wire I need to disconnect for the timing advance?
Note: One shop told me that my harmonic balancer 'slipped' a little.

Answer
Below heater housing on inside floor board look for a single tan and black wire with a connector on it.

But if the thing is slipped you will have to get the no 1 piston TDC and remark balance.

OK, only a code 12 (repeatedly).

I checked the timing as instructed - set at zero.
Then I manually adjusted the timing slightly for the smoothest idle I could get - no pinging or knocking. I did notice the heater was putting out a little better afterwards.

My vacuum was almost 18 at idle and nearly 20 at 2000 rpm. I used the PCV line to check the vacuum.

I did notice something about the timing flashes. At idle the light would flash pretty quickly, but when the rough idle cycle comes along, the flashing slows way down.

I looked at the injector spray pattern. Nice and even on both and no observed dripping.

Going back in time, I did replace the ignition module with one from AutoZone a couple of years ago. Could that be the problem? Or a weak coil? Bad coolant temperature sensor (AutoZone)? Maybe a bad distributor? :banghead:

If I manually depress the EGR valve diaphram, the idle drops and the engine briefly wants to stall. It seems to be working.

Code 12 is usually a pass test code all ok.
But.
You do not manual adjust timing.

Set it to the base time and let the pcm do the adjusting.

If vacuum is back up drive it and recheck gas mileage.

If I put the timing back to where it was, the truck runs very badly at idle.
I didn't have time to verify TDC so I adjusted the time slightly to obtain a smoother idle, which still isn't smooth.

It's only advanced about 2 or 3 degrees. Driving the truck runs smoother and the loud exhaust leak sounds mellower, and the heater seems hotter too.

Should I buy a spark plug adapter and use compressed air to see if a valve is stuck?

Set it at base time and forget it.
let the pcm adjust the rest.
Is it now running good or has gas mileage came up?
Is the vacuum back up good?

The vacuum only came up about 2" after I advanced the timing a couple of degrees. Seems that my balancer indeed has slipped slightly.

Gas mileage still bad, but the truck runs slightly better, except when cold.

After our upcoming cold spell, I'll block off my EGR valve and see if the truck returns to normal idle. If it does, then would that be the culprit? Or carboned up intake manifold passages/valves?

I need some magic repair in a bottle I feel....

Also, I was wondering about the cans of RESTORE. Do they really work or just marketing hype?

what i would do is just dump a bottle of sea foam in the tank with about half a tank of gas, exhaust my smell worse for a while but stuff does work, also i have found and i'm not sure if this is possible with the z motor but i trickle sea foam directly into the throttle body with the engine hot and running then turn it off let it sit for a while then start it up and drive it like hell, should have lots of white smoke come out of exhaust which shows its working.

As said sea foam may help.
But the cans of restore are SNAKE OIL.
You need to work on getting the base time set right.
If the balancer has sliped I would replace it or at least remark it to TDC.
Good Luck
MT

About 2 years ago, I had similar symptoms with a rough idle. The repair shop replaced the Idle Air Control Valve. Could this be the culprit?

When its cold, for about 1-2 minutes, the engine is in open loop. In this case, the engine's ECM does not use the 02 sensor for feedback for air/fuel ratio. Once the engine gets warm after about 1-2 minutes, the system then switches into closed loop and uses 02 sensor feedback for fuel managment. Now this may not indicate a bad 02 sensor, it could be a varity of things from a bad temp. sensor causing the ECM to over compensate to a bad fuel system, in which the ECM usually cannot compensate for the problem. Whatever the case, the ony real way to check 02, coolant temp sensor etc, is to have a scanner on there and watching what things are doing. If I were to guess though, If the fuel system is operating properly, no leaks, good spray pattern from both injectors, good ignition, temp is getting up to where its suppose to be, I would start at the 02 sensor and work my way up. I also wouldn't rule out the ECM itself too. Either way, you need a scanner on there to see whats happening first and go from there.

Also to add, easy way to check EGR valve, is to disconnect the vaccuum to it and go for a drive. This method would only be good though if you have a good idle to start with.

I had fuel pressure tested. It was 11-12 PSI. The EGR valve and Solenoid was replaced by the dealer ($361.00 mind you - OUCH!), The injectors were replaced (special order flow tested for $100.00 each + Labor), I replaced the plugs, wires, rotor, and distributor cap.

Interestingly though, the dealer told me that the EGR solenoid was 'shorted' as to why it had to be replaced. I replaced the EGR valve about 2 years ago with one from AutoZone - and it worked great! I'm not sure why the dealer replaced it again. I still had some problems and the dealer told me 3 things: 1) the truck had high mileage on it, 2) buy a new truck, or 3) buy a new engine.

Every shop (4 of them now) keep telling me to buy a new truck. My truck is a rust bucket, but when it ran smoothly, it ran really good. 18-20mpg out of an old pickup truck is decent, even by today's mileage comparisons.

I need to keep this truck as long as possible due to financial considerations, but at least 3 more years or so.

About 2 years ago, I had very similar symptoms with a rough idle about the 2 minute mark. I took it to a shop and they replaced the IAC ($175.00).

Regarding a scanner. AutoZone doesn't have one for an ODB I computer. The only shops I know of that have the proper scanner charge between $75-$85 per hour for diagnostics for which I paid for diagnostics to just be ripped off.

I KNOW my truck. I treat it with care and regular maintenance. I treat it well and so far, up to recently, it has returned the favor.

I suppose to test the computer, I would just need to unplug one of the sensors and see if a code appears.

You mentioned an open/closed loop mode. The truck runs rough when it's both hot or cold, but mostly when it's cold. I do feel the RPM's go up and down, but it's slight - like the computer is trying to compensate for something.

I did notice on this cold (3deg) morning, that the heater wasn't as hot as it usually is, even though the temp gauge claims the temp is between 190 and 200.

The haynes manual won't give me any resistance values for any of the sensors or how to accurately test sensors. It mentions back probing this or that. What is back probing?
I miss the old GM & Ford factory manuals that gave exact information.

Is there a way to force the computer into closed loop? :popcorn:


What should the O2 voltage readings be when cold and when hot?

I can't find a good repair shop that is willing to properly diagnose my truck.

Anyone live close to Cincinnati, OH that can give me a hand? (cheaply please)

Did you ever get the base timing set right?

For good repair information.
I would suggest a All data sub.


http://www.alldata.com/products/diy/index.html

ON FINDING A GOOD REPAIR SHOP.
Check around Ask around
Word of mouth.
Family friends coworkers neighbors business people and delivery people.
Mailmen and parcel delivery people get around and notice a lot of stuff.
Even a good parts house knows what shops are good and not good.
Chamber of commerce and better business bureau and city hall.
If you find a good referral to a repair shop go look them over and talk to them.
Look for a clean looking busy place with nice people running it.
Ask a few questions and ask about their qualifications and training.
Not all places have trained tech/mechanics.
If they do not find a place that does.
Even all dealers do not have all trained techs/mechanics.
MT

Can this help me find the problem?

http://www.autozone.com/az/cds/en_us/0900823d/80/12/39/f6/0900823d801239f6/repairInfoPages.htm

Can this help me find the problem?

http://www.autozone.com/az/cds/en_us/0900823d/80/12/39/f6/0900823d801239f6/repairInfoPages.htm


Yes it should help.
It is Free All data information.
But a lot of good info in the AZ site.
AZ owns and sells all Data.
If you do not find all you need there and the help you need there.
The get the online All data sub.
Only 24.95$ us plastic a year.
http://www.alldata.com/products/diy/index.html
MT

This morning, before I started the truck, I unplugges the IAC. I proceeded to start the truck and ran it for about one minute.

The truck ran a whole lot smoother. The idle started to pick up a little too much for my taste so I shut off the truck, plugges the IAC back in, then restarted the truck.

The truck ran poorly as it has all along.

I ran the truck for about 2 minutes or so, then shut it off. I unplugges the IAC a second time and restarted. The idle was a lot smoother, but still too high for my liking.

I shut it off again, reconnected the IAC, and restarted. I then went on an errand, then went on the interstate for about a 20 mile drive one way.

I parked the truck for about 1 1/2 hours or so then returned home.

The truck ran smoothly on the interstate, and some of the rough idle was gone when I arrived home.

Could it be a bad IAC?

Also, My dad took me around looking for another truck. We saw a 2000 Ranger 4x4 extended cab 6 cylinder with 90K miles on it for $9,000. Is that a good price? The exterior looks like it has been taken care of and the owner of the dealership said the truck was sold to his brother's brother-in-law and subsequently traded in on a new truck. Of course he mentioned this 'fact' 3 different times in the 1/2 hour we were there.

Additionally, we looked at a new Ranger 2WD I4 for about $15,500 sticker.
Would that be a good truck and a good buy??

Out of curiosity, about how much is my truck worth? The truck has a lot of rust, but generally, it did run quite well.