i have a 90 coupe, vin code 8. The engine was rebuilt last year and all new parts installed. Car was taken out and test drove, and when stomped on the gas the motor bog'ed out. As of todays date i have replaced all the injectors 4 times, all 3 cats, replaced pick up screen on fuel pump and cleaned, tank cleaned and all fuel lines blowen out, replaced fuel filter and fuel rail cleaned and blowen out, new fuel hose where needed, o rings where needed, new coil and related parts where needed, new distrubiter, in short all parts have been replaced over and over, and this motor still has a bog in it when you stomp on the gas. Car has just come out of the chev garage with more new parts again and motor still has bog. My next step will be to readjust the valves but at this point i dont really think that will get rid of the bog,

any ideas would be great as this car is now not far from the junk yard if it will not run right

thanks
Gary
gpardue@charter.net

 

It's a long shot but just wondering -- have you disconnected the EST plug at the distributor and checked the base timing with a timing light?

 

Yes the timing was reset at the chevy garage and told me it was not off, all the sensor have also been replaced and reset

 

Is the car bogging down only when you punch it? In other words, if you take your time getting into WOT, will the car accept that but just won't allow you to floor it?

If so, have someone who has a scan tool with a capture mode, "sneak into" a WOT condition (meaning don't drop the hammer but ease into WOT) and play back the capture to see what the injector pulse width value is when at WOT. Repeat the test when you drop the hammer and look at the pulse width value again for that condition. If the punched WOT width is significantly less than the sneak WOT width, the engine is being starved for fuel when you punch it. If the reverse is true, it's getting to much fuel when you punch it.

Assuming you can get to WOT slowly and the engine runs OK in that mode, you need to find out if the punched conditions are different as far as injector widths go so you can at least see if the engine is starving for fuel or getting to much when it bogs down. (There will be a little difference between "sneak" and "punch" but it should not be a whole bunch.)

Also, bear in mind that the injector pulse width is an instantaneous command from the ECM and is only an indicator of what is being commanded. The regulator has to be providing enough fuel or it doesn't matter

I'm assuming that the Chevy dealer will have checked that first thing. Only problem is they may not have checked it under the conditions where you are experiencing the problem. You should make certain they check the pulse width AND the fuel pressure under both the conditions I'm talking about above.

 

The only difference between swatting it and easing in is the tps did you check it.. It used to calc rate of change for the acellerator pump function IE enrichment fuel..

 

the problem only happens when you stand on the gas, if you bring the car up to speed slowly every thing seems to be fine, i also have a fuel pressure gage on the fuel rail and it reads a steady 35# of fuel pressure.

as a further note i got the injectors from fuel injector connection in Cumming, GA., Jon Banner
2 set from him , Borg injectors

also the person who rebuilt this motor probed the hell out to the injector wires, could it be possible that one of them is shorting out under a load

 

So do you know for sure that the Chevy dealer monitored the fuel pressure and injector pulse width under the load conditions where the problem occurs as well as when easing into WOT? They specifically told you they had done this?

A situation where you can ease into WOT but can't punch into WOT is a clue that needs to be explored.

If the TPS is hanging up as suggested above, you will see that in the injector pulse width and the block learn values (hence the need to capture things with a scan tool.)

The ECM would interpret the (faulty) low TPS voltage as being a signal to deliver the fuel at a lower rate (shorter pulse width) for an instant. Meanwhile the throttle body will be dumping gobs of air into the plenum and your car is going to bog down as a result of a super lean mixture.

Under these conditions, the ECM is in the best emission mode (14.7:1) and, because the TPS is sticking and the ECM doesn't think you've gone WOT, it will try to keep that ratio. The block learn value will be something other than 128 as the ECM attempts to compensate for the lean condition caused by the throttle body being wide open yet seeing a faulty TPS value. (At 90% of WOT and above, the block learn is frozen at 128. The ECM is out of the 14.7:1 mixture mode and is allowing best power mixture. Under all other conditions, the integrator circuit in the ECM will cause the block learn values to hunt around as required to keep the mixture at 14.7:1. A WOT should cause the block learn value on a stock L-98 motor to be frozen at 128 and not hunting as long as you are at 90% of WOT or above.)

It may well be that the TPS is hanging up only when you punch it but if that's going on, you'll see it as a different (narrower) pulse width value when you first punch it as well as varying block learn values because that is what the ECM thinks you've commanded since the TPS is lying to it.

A scan tool with a capture mode will grab the inj. pulse width, block learn value, TPS value and the spark advance value for the few seconds centered on when you have the problem, This, coupled with monitoring the fuel pressure under load, is what you need to do to understand what the engine is doing by comparing the "punch mode" to the "ease mode."

You've obviously spent a ton of money with this dealer, they should thoroughly check this for you.

 

I will take this info to the dealer on monday morning and talk to him , will get back to you later in the day on monday

 

c4-90-41001

i took the info you stated down to the chevy dealer and they said they had not preformed the test you mentioned, they told me to bring the car back and the would check it out. will let you know what the results are
Gary

 

I'm glad they are going to stop throwing parts at the problem and do some diagnostics.

It may turn out that the problem is the TPS as ddahlgren said above. It may be something else. The thing is, you need fuel, air and spark in the proper proportions and at the right time to make the engine run correctly and capturing basic operating parameters with a scan tool plus monitoring fuel pressure for both easing into WOT and hammering into WOT will provide a comparative assessment.

When they capture (if they do it correctly,) they'll get everything that's monitored, not just the things I suggested, so insist they do a full data capture.
Once they do this for both conditions, they'll have quite a lot of data to look at.

Just one word of caution though: A scan tool only reports information and it's up to the technician to interpret it. If the TPS is taking a powder when you stomp on it, that will show up clearly but if it's OK and (for instance) the engine's timing is going bad at that instant or the engine is running rich or lean at the instant, you're at the mercy of how good the tech is at figuring out scan tool data and relating it to what could be causing the anomaly.

Still -- since they admit they haven't tried to capture raw data to help solve the problem up to this point, just getting them to do what they should have done long ago is a step in the right direction and a small victory.

Let us know what they find.

 

c4-90-41001
took the car back today, they ran the scan test and i have in a zip drive saved in a .sur file ext. i have tried to open it or find driver to open on my comuter, but nothing works, i think it will only open on a chevy repair shop computer, i would be glad to send you a copy of it to look at if you can get it to open as it is all greek to me. the shop forman there would like to talk to you on the phone if it is ok with you and would give me your phone number.

thanks
Gary

 

I wouldn't be able to open it either, Gary but I don't mind talking to them.
I'll send an email with my phone number to your charter.net account.

 

While I am waiting the results of further testing on my vette, I would like to mention Jon Banner from FUEL INJECTOR CONNECTION. This guy is more than willing to go the extra mile for a guy to help him out with a problem, In my book he is highly recommoned and if you can support his business we should....Thanks Jon

Gary Pardue