99 Tahoe 5.7L vibrates and looses power

[dzzyd1970]

Hi Folks- my 99 has some issues.. symtoms are slight vibrations and loss of power when accelerating uphill at freeway speeds then right when gears drop to overdrive.. however, its not a chassis vibrations its appears to be an engine tuning vibration almost like the engine is missing. Also, when this happens the "Service engine soon" light flashes and stays on then goes off a few days later. I do need a new gas cap as the seal is bad. The book mentioned I might have a cylinder misfire? She runs great all other times.. Any ideas what this can be? Thank you so much!

 

[Abmills]

I'm having a similar problem but mine happens in the morning. It runs like crap then in the afternoon it is fine. Already changed the plugs, wires, cap and wires. Next step is looking for a vacuum leak. I hope someone else with this problem replies with a solution.

 

[dzzyd1970]

(abmills)Yes, I have heard from other threads that I could be dealing with fuel inj issues that adding sea foam at 1/4 gas tank could help. also, o2 sens, cat then the issues you mentioned.. I just hope im not hurting anything...

 

[lesterl]

Seafoam the intake real good also.....

 

[dzzyd1970]

Lester, can you buy seafoam just about anywhere? I plan to check the cables, plugs, pretty much do the obvious first before I add the foam.. thanks for the tip..

 

[lesterl]

Seafoam is available at walmart, autozone, orileys, etc..... If they sell oil, pretty good chance they got seafoam also, will usually be in the supplements area (like where the rest of the cleaners/ stopleaks/ etc reside.

 

[dzzyd1970]

tried the Seafoam through 10gal of gas and no change.. will move on to next steps.. should I try o2 sens? what else should I try?

 

[lesterl]

Have you pulled the codes yet?

I like to seafoam down a big vacume line while engine is running in CL state...

 

[dzzyd1970]

No, I havent.. I need get the reader from my buddy. sorry, very limited to what I can do.. But I can read the codes probably this weekend.. Will the codes still be in Memory if the "service engine light" is now off? It only comes on some times.. then goes off. when I look in the Tahoe mnl ( I know lame) it says Misfire on cyl #1.. Im not sure how to run the seafoam for vaccuum line while eng is running is CL state. I also noticed from day one that the idle is about 1/2 and was thinking I should take out the throttle body and clean? I got the instructions on that and was thinking about making that a weekend project this week plus the codes... But, give me your next steps you would do.. thanks Lester~!

 

[lesterl]

What is the idle in RPM?

CL is Closed Loop (engine warmed up)

Codes will be in memory up to 50 key cycles after setting on most codes.

If you havent read the codes yet, how do you know the "Tahoe mnl" says misfire on cyl#1?

To Seafoam down a vac line, run engine to temperature, (I like the brake booster vac line) slowly unhook the line so as not to stall the engine, then slowly pour the seafoam down the intake untill about 1/2 is gone then dowse it pretty good to try and stall it, if it doesnt stall, turn it off quickly and let it sit 10 min or so, then fire it up after and run it around the neighborhood killing off mosquitos.....

 

[dzzyd1970]

Hey Lester, so the book that came with the tahoe indicated that when the service engine light flashes then stays on that it could be the several things.. one of failures one "misfire on cyl #1. I will double check today that I read that correct.. and I will also get my buddies code reader and read the codes properly. I will also try the Vaccum line with Sea foam. Thanks!

 

[lesterl]

Flashing CEL is basically a harmful condition that can do some damage if you dont stop the engine immedieately... Misfire on any cylinder should flash the CEL.....

Get that code reader and let us know what you find this week end...

 

[dzzyd1970]

Hey Lester, ok.. I will get he code reader tomorrow when im in the city from my buddy.. Also, I just noticed I have Seafoam that you put into the gas engine not the intake.. I guess I need to get the intake kind? Anyway, I will get the codes pulled and report back the findings. Thanks!

 

[lesterl]

The other use for the Gas Seafoam is thru the intake, should be on the bottle IIRC.....

 

[dzzyd1970]

Hey Lester, ok pulled the Codes.. PO300 was all that came up. Generic definitions in the book i have told me its multiple misfires across all cylinders? Is this bad news?

 

[lesterl]

Random misfire... I would check for wear on the distributor drive gear on the bottom of the distributor.... Fuel pressure? What are the fuel trims?

 

[dzzyd1970]

Hi Lester, not sure how to check fuel pressure or fuel trims. I know I can figure it out.. Since I have only had this Tahoe for a little over a year. I have no problem swapping the Distributor, Cap, Rotor, Cables, Plugs.. Fuel Pressure what would that tell me? Fuel pump or filter is bad? Again, thanks for the help..Much appreciated. again, as a stated previously the idle sits at 500

 

[lesterl]

You would need a guage to test fuel pressure. Pressure checks help verify pump and injector systems are working fine.

Fuel trims will require a scantool, this "measurement" is basically what the PCM sees from the O2 sensors and changes the fuel mix.... either adding or subtracting fuel....

Might also get some MAF sensor cleaner and clean the MAF sensor.....

 

[dzzyd1970]

Hi Lester, yesterday I noticed one other symtom. when acelerating uphil on the freeway and and as soon as the service engine light started flashing and engine started vibrating I noticed the smell of gas coming through the vents or from outside.. there was no other vehicles in front of us.. Does this change the direction I should go in? Before I do start the fuel pressure check and MAF check I thought I would run this by you first... thanks!

 

[lesterl]

I would check the fuel pressure, but suspect you have a possible bad spider injector or a FPR leaking.....