Lets see how many of you can diagnose this. (I already fixed it but I figured I'd throw it in here as a learning experience for others) I'll post the fix in a day or 2

So I get done with a service call yesterday (Weds). My truck is giving me probs starting. I finally manage to get it started. But at low RPM it's running rough then stalls. At hwy speeds everything is fine, but at lights
I have to keep the engine revved above 1200 RPM or it just stalls.

Freaked me out to hell and back being 30 miles from my tools and this happens. As soon as I examined the symptoms in my head I realized what it was. And no SES codes were set. 60 bux later things were back to normal..:)


Feel free to ask me about other symptoms, again just wanted to throw this out here for giggles and see what others come up with. Like I said, it's already fixed.

A fuel injector clog? What was it?

Idle Air Control Valve

Bad 02 Sensor, or EGR...
TPS or MAF or MAP or LOW Battery Voltage, Bad Ground,, Coil, Fuel pressure regulator.....

Understand that these are the things I would CHECK FIRST, And not just go on a parts buying spree

Keep on goin guys.... Yer gettin there... And when you do, you need to figure out why it caused the symptoms....:wink:

(Point of the excercise is to figure out cause and why it causes the symptoms as opposed to just "Throwing Parts and Money" and a problem...:) Welcome to diagnostics 101 gentleman (and occasional ladies):icon16:

Bad cat?

Ditributor cap or wires.

My first guess would be a vacuum leak since it's running OK at higher RPM's. But, that wouldn't cost $60 to fix. Unless it possibly had to do with a MAP sensor....

I'm going to agree with srbianats and go with the dist cap.

Was the cable to your MAF unplugged? ha.

He doesn't have a MAF system.

Not a vacuum leak or the rpms would be higher.

Stalling and lower rpms almost makes me thing a PCV valve was plugged.

That or the fuel filter is clogged.

It can't be a sensor seeing it would trip a SES light. I think it was fuel or PCV or a restricted CAT.

I'm sure that it's going to be the last thing that we think of otherwise he wouldn't have started a thread about it. Even so, I'm going to guess a shorted battery, at low rpms the alternator could supply enough power to deal with the battery and provide enough for the electronics, injector, and fuel pump to function properly.

Id say operater malfunction?....seen as how it took 60 bucks...he prob got drunk on?...lol

Nope. nope, and ,more nope...I'll post the fix tomorrow... Keep on guessing though!

fuel pump

fuel pump

$60 would be a hell of a deal for a fuel pump.... :)

fuel pump

For 60 bux? I wish...:screwy:

Serpentine belt slipping. *Would give problems starting. *At low rpm engine would run rough and/or stall from low voltage. *At higher rpm the alternator output increases. *Charging system failure will not trigger SES light. $30 bucks for a tensioner and $30 bucks for the tools to change it! That's my story and I'm stickin' to it!


Rlith, You're asking us to diagnose a problem bypassing the first and most important step in diagnosis, "A good visual inspection". ;)

Bad Battery. You didn't say whether you had to Jump it. Alternators are $125, Fuel Pumps $100, Fuel filters $25, PCV Valve $5, About the only thing left is the Battery for $60. It was bad due to low water.

I would say a fuse/relay problem, more specifically the fuel pump system.

I agree with old master, a good visual inspection would help.

egr valve, $60 labor to r&r it.

Bad Battery. You didn't say whether you had to Jump it. Alternators are $125, Fuel Pumps $100, Fuel filters $25, PCV Valve $5, About the only thing left is the Battery for $60. It was bad due to low water.

My Redtop is less than a year old, but interesting guess!

egr valve, $60 labor to r&r it.

I'm gonna pay a garage to change my EGR? :icon16:

Serpentine belt slipping. *Would give problems starting. *At low rpm engine would run rough and/or stall from low voltage. *At higher rpm the alternator output increases. *Charging system failure will not trigger SES light. $30 bucks for a tensioner and $30 bucks for the tools to change it! That's my story and I'm stickin' to it!


Rlith, You're asking us to diagnose a problem bypassing the first and most important step in diagnosis, "A good visual inspection". ;)
When it happened I did do a visual inspection of everything, no probs showing... Just the symptoms... Took me a few mins of wracking my brains through my mental diagnostic flow charts.. Belt is fairly new, tensioner is less than a year old...(My god the amount of stuff I've replaced on this truck, heh) I'll post the soultion at lunch time

^ good, i'm very curiouse.

When it happened I did do a visual inspection of everything, no probs showing... Just the symptoms... Took me a few mins of wracking my brains through my mental diagnostic flow charts.. Belt is fairly new, tensioner is less than a year old...(My god the amount of stuff I've replaced on this truck, heh) I'll post the soultion at lunch time
You have to keep in mind that this is your truck that we're talking about. You know it better than anyone, you know the weak spots and issues that it has.


Maybe an exhaust leak? While sitting still the exhaust vapor built up under the hood, robbing the engine of O2, causing it to choke out and die.

Just another guess. After driving a while, you realized that you were actually driving someone else's Blazer that looked just like yours. When you returned to give the Blazer back, and get your Blazer, the police were there and gave you a $60 ticket for joyriding. You got in your Blazer and it still ran fine. Problem solved. :thumbsup:

^ that MUST be it,.....

Actually blazee that's come close to happening to me on more than one occasion.... I have come out of someplace and because I tend to walk with my head down, all I see is a teal blazer/jimmy and my brain is on auto pilot... I sit there for a moment going "What the hell, my keyless isn't working) And try to use my key... After about 10 seconds I realize, shit, it's not my truck!!! My truck is over there! And I feel like an idiot and hope to hell that the owner didn't see me trying to get in "their" truck...:banghead:

LOL, thats pretty funny. I'm sure it happens all the time though.

I have a teal blazer,,,

$60 would be a steal for diagnosing and r&r an egr wouldn't it?

Feel free to ask me about other symptoms, again just wanted to throw this out here for giggles and see what others come up with. Like I said, it's already fixed.

I'm asking.

What are the other symptoms.

Here's my guesses;
Did you drop 3 $20.00 bills in the TB housing?
Did you shread your K&N filter Assembly?
How about the vacumn ball cracked?

(O2 sensor would/should have sent a code, EGR symptoms are similar, but the valve is more $$$$)

Actually blazee that's come close to happening to me on more than one occasion.... I have come out of someplace and because I tend to walk with my head down, all I see is a teal blazer/jimmy and my brain is on auto pilot... I sit there for a moment going "What the hell, my keyless isn't working) And try to use my key... After about 10 seconds I realize, shit, it's not my truck!!! My truck is over there! And I feel like an idiot and hope to hell that the owner didn't see me trying to get in "their" truck...:banghead:

Once I had to go pick-up something quickly, as I got back to "a" truck, I got in and not even thinking went to put the key to start it up, the key didn't work, I then looked around inside and realized, IT WASN'T MINE, my truck was in the next row in the parking lot. The truck was identical. So carefully, yet quickly, got out, took my package, got in my truck and drove away.

Once I had to go pick-up something quickly, as I got back to "a" truck, I got in and not even thinking went to put the key to start it up, the key didn't work, I then looked around inside and realized, IT WASN'T MINE, my truck was in the next row in the parking lot. The truck was identical. So carefully, yet quickly, got out, took my package, got in my truck and drove away.

Similar to, but different than...

My MIL was sitting in our Pontiac TransPort a few years back. Some lady comes up to the back hatch, inserts a key, and twists the lock a few good times and ends up breaking the locking mechanism. She just kept at the darn thing until the mother-in-law got out and asked her "what are you doing?"
"Ooops, I thought this was my Van" , and she proceeded to wander the parking lot looking for her own van.

Didn't find out until a few days later that we couldn't lock the back hatch anymore. It stayed that way for 5+ years (traded the van 2 years ago)

Ok guys... Biv343 was the closest when he mentioned the MAP sensor.

Here's WHY I knew it was the MAP sensor...

If it had been the EGR the truck would have kept idle but would have accelerated though the power band rough until WOT where it would smooth out most likely.

The symptoms indicated the MAP sensor and here's why...(I'm going to cut and paste)

The map sensor is mounted on or connected to the intake manifold to monitor intake vacuum. It changes voltage or frequency as manifold pressure changes. The computer uses this information to measure engine load so ignition timing can be advanced and retarded as needed. It performs essentially the same job as the vacuum advance diaphragm on an old fashioned mechanical distributor.
On engines with a "speed density" type of fuel injection, the MAP sensor also helps the PCM estimate airflow. Problems here may or may not cause an intermittent check engine light (light comes on when accelerating or when the engine is under load if at all), hesitation when accelerating, elevated emissions and poor engine performance, cannot hold idle, and rough starting. The engine will run with a bad MAP sensor, but it will run poorly. Some PCMs can substitute "estimated data" for a missing or out of range MAP signal, but engine performance will be drastically reduced.



My words suck, so this was easier. So at 1st eliminating the egr in my head, I knew it had to be the MAP sensor because at regular running speed everything was alright, Just would stay on at idle. So 60 bux later I swapped out the MAP at a local autoparts store to where I was (they were kind enough to let me borrow a couple of torx drivers) and I was on my merry way...:)

Sounded almost like what my 85 S-10 did after I rebuilt the engine this winter, except my problem was "installation malfunction". When I first started the truck after putting it all back together, the engine wouldn't idle at all. Found out after a bit of head scratching and Haynes manual reading that I had connected the vacuum line to the wrong vacuum port on the TBI, so the sensor was reading manifold vacuum instead of ported vacuum (or vice versa). Moved the line, and it's been fine since. Mine threw a couple codes though.

MAP sensor? Well of course that's the obvious answer, I would have said that, but I was expecting something unusual. :icon16:

MAP sensor? Well of course that's the obvious answer, I would have said that, but I was expecting something unusual. :icon16:

Sometimes the most complicated problem has the simplest answer.. :wink:

Mine is doing the same thing except it doesnt stall.
Here is a link (http://www.automotiveforums.com/vbulletin/showthread.php?p=4190187) to mine.

Mine is doing the same thing except it doesnt stall.
Here is a link (http://www.automotiveforums.com/vbulletin/showthread.php?p=4190187) to mine.

Blazer LT already gave you the solution

Blazer LT already gave you the solution
thats not what it is. MAF sensor is good, I bought a new one and it still is doing the same thing.

My truck is giving me probs starting. I finally manage to get it started. But at low RPM it's running rough then stalls. At hwy speeds everything is fine, but at lights
I have to keep the engine revved above 1200 RPM or it just stalls.

If it had been the EGR the truck would have kept idle but would have accelerated though the power band rough until WOT where it would smooth out most likely.

I would have guessed for sure it was the EGR. In your first post you said it was hard starting and stalled at low rpm's but ran better at high rpm's. I had the same thing happened to me and hooked my truck up to a scan tool and it said my egr was stuck at open 80% instead of closed to start. I cleaned it and it ran fine. So in your second quote here are you saying this is what would happen if your egr was stuck open or closed? And just for future knowledge do both the tbi and the cpi motors have map sensors? Because I have the tbi and had the motor rebuilt not too long ago but it still kinda hesitates on take off. Just wondering if that could be a cause on the tbi.

MAP sensor? Well of course that's the obvious answer, I would have said that, but I was expecting something unusual. :icon16:

I was going to say that, but wanted to give others a chance. :grinyes:

Anyways it's like saying "my truck won't crank" and give a few symtoms and then try to guess what it is. It could be a number of things. Some very popular, some not so popular.

bad belt tensioner

intake gasket

OK, I'd like to add to the map sensor story. I just returned from vacation pulling a 4800 pound boat behind my 93 s10 blazer. Its a 4.3 and the boat is as much as it can pull, maybe a bit too much, but it manages.
While backing the rig down the ramp to launch the boat, the car stalled and I had NO brakes! Never stalled before and I almost lost the whole rig in the lake. Anyway, afterwards the engine ran rough, and had trouble starting but ran OK at higer RPMs just as descibed above. I did get a code on the PCM however and it was for the MAP sensor. By the way, there are no power brakes at all when the engine won't idle; there just isn't enough vacuum to run them.

Anyway, I spent my $60 bucks at a nearby Advance Auto store and installed the map sensor in their lot. That fixed my problem at once and I drove away a happy camper--for about 6 blocks and the new map sensor went bad. I returned to Advance and they had to order a 2nd one and I installed it later, this time when the engine was cold.

The new map sensor seems to work, its been a week now, but the car doesn't run as well as it used to. I get an occasional miss at idle and it seems to hesitate at low speeds. I'm wondering if I have a junky sensor from Advance auto and maybe I should try to get another one from Napa--like Advance is going to give me a refund :(
or if there is something else to check.
I've pulled out and checked/cleaned the IAC and EGR valves and the plugs/cap/wires are 6 months old. I replaced the fuel filter yesterday but it didn't help; it just seems like the map sensor isn't working as good as the old one did.

I'd pull the upper plenum and check for washing...Check plug condition as well., The map sensor could be reading incorrectly due to too much gas in the intake. If that's clean, pull the ECM battery fuse for 10 minutes to reset the computer and let it relearn.

I think I have to go with fuel pump too.

An update...
Yesterday the car won't start when I tried to go to work. Fuel pump failed. This only happens when you fill the tank (somebody explain that to me) and have 16 gallons or so of fuel to deal with when pulling the tank out. Oh yea, GM really played nice with the fuel lines on the top of the tank too. No play at all in the lines and there is no room to get tools up there and unscrew the fittings with the tank in the way. Took me all day and I ended up breaking off the return line connection trying to unscrew the fitting.
Sigh..

Car works better now, but still has some unsteady idle; its not as bad, but not like it should be. I'll pull the plenum this weekend and see how it looks in there.