1991 L98-At WOT engine boggs down and nearly stalls.

[redvette91]

I am going crazy with this problem! It has been to 2 shops and neither can figure out what the problem is. When you stomp on the gas to WOT the engine boggs down and nearly stalls. It gets so close to stalling you think its going to any second and never does. There is no check engine light or codes. When gradually accelerating its fine and will rev all of the way up but if you stomp the gas thats when it does it. The car has 19K on it, garage kept, never seen rain or snow, not a spec of rust and every part looks like it did when it came from the factory. It feels like it has a small misfire at idle and clears up when you gradually accelerate. When in gear it gets up to about 15 and then it feels like you turned the key off but doesnt completely stall out. If I hold the gas to the floor it will backfire once after about 15 seconds and then i let off and scratch my head some more. It seems like plenty of fuel and pressure is getting to the rail. I unplugged the line after the fuel filter and turned the key and seems like there is plenty of fuel coming out. The shops said they checked the pressure and had it on the scanner and check all the sensors, ignition and everything they could possibly think of and they say its all ok. I just dont get why it revs up fine when i gradually do it and every time i stomp on it, it immediately boggs down to the point where you can barely tell its still running. I thought it would gradually bogg down but no, i can rev it up to 4000 and then stomp it to the floor and feels like you turned the key off. One shop also replaced the computer and that didnt help. Then they thought the front cats were clogged so i ordered a y pipe with no cats and still nothing. ANY help would be greatly appreciated!!! Thanks in advance.

John

 

[Schrade]

Feels like a misfire at idle? Does it lope idle? Up and down rpm's?

Get a fuel pressure test done:

Either buy a fuel pressure guage, and do a static and dynamic pressure test, or pay a mechanic to do THAT ONLY, and post back results.

Static is 'motor off' fuel line pressure, to see 1) what it goes to, and 2) how long it takes to bleed down. I think this is what vetteos was gettin' at. If the injectors let the fuel pressure out, leaking into the cylinders, you'll have black smoke at a cold start in the am. Just like a garden hose with a leaky sprayer. With a new pump, the pressure is probably good, but you need bleed down time. Hope you still have your old pump - it's probably still good, and you can put it back, and buy the fuel pressure gauge when you return the pump..

You probably do not need a dynamic pressure test (motor running pressure), since starting up is the problem. It still is a good thing to know.

See youtube for fuel pressure test vids.

 

[redvette91]

no it doesnt lope idle. it just shakes like it has a mis. i ordered a pressure gage so here in a few days i will know whats going on with that. like i said if i give it gas slowly its fine and as soon as i stomp it, it takes a crap and you can barely tell its still running. i would think that if it was a fuel delivery problem it would slowly bogg down and not within a snap of a finger.

 

[Schrade]

Good. Don't spend any cash till you get FP tests.

 

[gw962]

no it doesnt lope idle. it just shakes like it has a mis. i ordered a pressure gage so here in a few days i will know whats going on with that. like i said if i give it gas slowly its fine and as soon as i stomp it, it takes a crap and you can barely tell its still running. i would think that if it was a fuel delivery problem it would slowly bogg down and not within a snap of a finger.

I hate to sound like a broken record, but have you ohm'ed your injectors?

 

[vetteboy86]

There are three variables to the equation, air, fuel, and spark. Checking fuel pressure is a must. Do you know how a fuel pressure regulator operates and how to check it?

 

[dtl]

Fuel tank pickup has a "sock" or strainer on it. If you have picked up some dirty fuel over time that sock will plug up and collapse on WOT.

This would show up as low fuel pressure on WOT. Have to pull the fuel tank pump and pickup assembly out of the tank.

 

[vetteboy86]

Fuel tank pickup has a "sock" or strainer on it. If you have picked up some dirty fuel over time that sock will plug up and collapse on WOT.

This would show up as low fuel pressure on WOT. Have to pull the fuel tank pump and pickup assembly out of the tank.

I've never heard that before. If this is actually the case, it isn't to difficult to pull the assembly out. Just some patience and time.

 

[redvette91]

Well guys, after a long weekend of work and pulling my hair out, this is what I have come up with. I checked the sock at the tank and its fine, I did all types of FP tests and thats good, I checked the volume of fuel and thats all good, and I checked the timing to make sure there wasnt a crack in the magnet in the distrubutor and thats good. The only thing with the fuel that I havent checked is the injectors yet, with doing everything else I was about to go nuts so I had to take a break haha. I just dont get this, I have NEVER run into a problem like this that I havent been able to figure out atleast within a day or so on ANY vehicle. FYI to all, I was a mechanic by trade for 6 years until a year ago when I got a govt. job. How do you suggest checking the injectors and what are the readings supposed to be? I dont have access to computer diag. equipment anymore which really sucks. I was also thinking if it was the injectors wouldnt it just be a cylinder or 2 maybe 3 that go down when stomping the gas and going WOT? The car instantly craps when I stomp it, every cylinder goes down at the same time and its so bad that I can barely tell its still running, I hold it down for about 10 seconds and try to get any more clues and when I let up it goes back to running almost normal with a slight miss.

 

[gw962]

you'll need a Volt/Ohm meter to do the quick check on the injectors.
Pull the connector from each injector and place the DVOM leads on each leg of the injector. They should all ohm at 16.5 +/- .5.
Since it will idle, also check them hot and post the readings. Odds are you have several that may be below 10. I had 3 that were in the 4 to 6 range, so total replacement was in order.

 

[G Winter]

Will throw some things at you, never know.

Did you check the TPS at wide open ? Maybe you have a bad spot at the very end.
I think the EGR is a positive pressure, opens with aid of exhaust pressure, some freak that the exhaust is opening it causing lean out ? Unhook and plug vacuum line to it, and then try it.

Could be a spark problem,,, Coil ? Light load requires less voltage then heavy load, harder to jump the spark at higher cyl pressure than at low pressure.

Glenn

 

[vigman]

GAP!

Do you KNOW what the plug gap is?

and

I would take it to a muffler shop & have the system back pressure tested...

Kinda sounds like a cat starting to go buh bye

 

[redvette91]

FUEL PRESSURES OK

Ive done it all. Even pulled the pump...removed the sock entirly. Coil and dist. is OK. GM bulliten out on the dist. shaft but all is as it should be. Done everything except testing the OHM's on the injectors. Will try to do that tonight. The engine still bogs at part throttle and wide open throttle. Next step.....about 5 gallons of gas and a match....LOL.:L Ive raced cars and built engines all my life and have owned 10 "REAL" (non computerized junk) Corvettes in my life. This thing has me stumped.ad

 

[gw962]

You can check your exhaust backpressure if you have a vacuum gage. Unhook your air injection check valves on the exhaust manifold and, using a rubber conical adaptor on the vacuum gage hose, insert it into the pipe. At idle, you should show 0 psi and no more than 2 psi, IIRC, at 2000 rpm. Check both sides, if you have pre-cats.

 

[redvette91]

CATS

Already check the cats first. Removed the pipe with mini cats and installed a pipe from Mid America without cats. The larger one in the rear is good to go. That was one of the first things I checked and Ive had the car at a friends gas station on his equipment. No check engine light has showed. ECM was replaced and coolant sending unit was repalced. Pressures at guage are 38 at idle and jumps to 50 at the stab of the gas and settles at 48 under load. Volume is good. New plugs (UGH) good thing the car is red as my blood wont show. No water in gas, filter is clean. What now?

 

[Schrade]

38 at idle is on the low end of acceptable. Regulator might have a weak diaphragm spring, or the diaphragm could have a leak, allowing fuel to get pulled into the regulator vacuum line. Pedal down makes vacuum to the regulator momentarily bump fuel pressure for acceleration enrichment.

Check also the vacuum hose to the regulator for strong fuel smell.

Did you note pressure bleed-down time with motor off?

Check static timing too. Spin the crank with a breaker socket to the 0 mark on the damper. Put a piece of tape on the distributor cap, and run the tape to underneath the distributor base. Carefully cut the tape so that you can pull the distributor cap, and see if the rotor has just passed #1 wire contact in the cap, on the way to #8. Make SURE that # 1 is on the squeeze. If the rotor is at #6, then #1 is at blow, not squeeze. Spin the crank one whole rev.

 

[vigman]

This is bit of a bender

How do the plugs/tail pipe look after a boggy run????

Are you running the stock cap/coil?

I went to a high output coil / trigger.

Is the TPS properly set?

Is it the STOCK ECM or aftermarket?

Is the engine ground strap good?

This has me going!

Vig~

 

[ken mohr]

Good. Don't spend any cash till you get FP tests.

I agree. Had a local shop working on a vette. Told them the fuel pump was bad and he said they checked the pressure and it was fine. 19#. Told it should be 40#. To cut it short $1000.00 later the changed the pump. Now 40# and runs great. Ask what the pressure is.

 

[redvette91]

OK....Here it is.......if anyone has the same problem #1 check your fuel pressure (mine was and is good). # 2 make sure your cats are not plugged up.....# 3 last thing I checked after "dicking" with it for almost 3 weeks and having someone who I THOUGHT knew what they were doing work on it (should have done this first) I did an easy check with an OHM meter....check the injectors FIRST. Mine were bad (half of them) at 8.3 to 8.7 OHMs (OEM injectors are 16.5 +/-- .5 OHMs). With all of the talk about junk OEM injectors on the various sites on the L-98 TPI engines I should have checked that first as its an easy job to check the required OHMs. The hardest part is replacing them but if you have a repair manual (and can read) and can tell a tork socket from a wrench you can do it yourself. While your in there you might as well install an adjustable fuel regulator. Damn late model computer cars. I want my 435 HP 67 car back. Screw the gas milage.

 

[Toms007]

Did you Ohm the injectors? Sounds like you have covered nearly everything else. Check the injectors and if they test OK, go back and check everything else..again. Don't overlook anything, new parts can be bad....ask me how I know. :L