da_gizm0

ok i hope u guys can help me..my 99 sierra is having a problem starting....it starts fine cold and hot...but if i let it set for like 30 mins or longer it just doesnt start right...it turns over for like 2 secs and then starts...it also surges sometimes when im at a light...the rpms drop to about 100 then the computer gives it more gas and it pulls me forward about 3 feet even when braking hard. its got 62,000 miles on it. anyway the first thing that i replaced was plugs and then i replaced the 2 o2 sensors that are after the cats..but the ones before the cats have a different style plug...any1 know where to get these o2 sensors, after i replaced the first 2 it doesnt surge anymore..but it still just doesnt start right.will replacing the other 2 sensors correct my problem? any help would be appreciated

da_gizm0

ttt

ThunderZ28

iTrader: (7)

I THINK YOU CAN GET THE FRONT TWO AT O'RIELLY'S THEY GO FOR ABOUT 105 DOLLARS. MAY HAVE TO ORDER CANT REMEMBER IF THEY HAD THEM IN STOCK OR NOT. DON'T KNOE IF IT WILL FIX YOUR PROBLEM OR NOT, BUT THOUGHT THIS INFO WILL ANSWER HALF YOUR QUESTION

Procharged LM7

ever changed the fuel filter?

L82gofast

iTrader: (1)

Procharged has a good question. Is it setting a code?

BigKID

iTrader: (5)

I seriouly doubt that the rear O2 were you serging problem since the do not contribute to fuel metering. I also doubt it is your front o2 either unless it is running bad all the time. My bet is with the fuel filter too.

2ksleeper

Based on the problem you described, If I were you I would check the IAC valve on the throttle body to see if it is sticking. But I believe you wasted your money on the rear O2 sensors if your truck wasn't throwing a code for them because the only thing do is check to see if your cats are running correctly, the front O2's are the ones used to adjust fuel trim. I would also check your fuel filter, it is a $9 part & if I were you I replace it every 30k miles. As far as a good place replacement parts online is (rockauto.com) I just replaced my front O2's on my truck for about $95 shipped.

CMNTMXR57

This is actually untrue. Albeit it is a different car I have had this problem plague it for nearly he entire life of the car (it now has 120k on it). It is a 96 Monte Carlo Z34. It has the troublesome "X" motor which is a VERY tempermental engine.

What would happen was that after a WOT run, the next time it came to idle, if it didn't surge, it'd die. It'd also die if you put it into park. Upon the next startup, it would surge as the PCM tried to re-learn an idle but it couldn't because of the codes set. Until those codes were cleared (after 3 successful consecutive operating cycles or with A-Tap/Tech2) would it return to normal.

The codes I got were a;

P0131 = Slow B1S1 O2 sensor response.
P0410 = Secondary A.I.R. failure

And an EGR code occasionally.

I first tackled the pre cat O2 sensor which is a bitch on this car. It requires the top of the motor to be removed, coils relocated, engine "rotated" forward and then you can still barely get it. I replaced it with a Napa/Echlin unit which I later found after putting it all back together, it didn't work.

I waited on the Secondary A.I.R. failure because I knew of a pending recall on it that would save me several hundred. Finally GM issued it and I took it in. It helped exponentially, but the problem still persisted and ONLY threw the P0131 slow B1S1 O2 respone code. Right away I knew it was the tempermental "X" motors tempermental PCM requiring a genuine GM O2 sensor and two weekends ago, I repeated the process I mentioned above to replace it.

BINGO!! Car runs like a champ! I ran it about 20 times in my favorite industrial park at WOT and it idles and runs like a champ. Putting it into park is as smooth as can be. Put it back into drive and it drives off with no hesitation as it did before. I look forward to driving this thing to 200k now.

So yes, a pre cat O2 sensor CAN cause this much havoc. If you get the similar codes and scenarios I did, I would look into it.

da_gizm0

no the engine light isnt on so i dont think its throwing a code..im goin to change the front o2's soon, its wierd that the back o2's helped some if all they do is monitor the cats. im also going to change the fuel pressure regulator, ill change the fuel filter also. isn't it located back by the tank?

Scream

iTrader: (6)

If there are no codes, I wouldn't bother changing the O2 sensor. Don't just throw money at a problem and expect it to get better.

I wish that I had a better solution to offer to you, but I really feel like you're headed in the wrong direction.