rshanholtz

My 86' 951 with an 89' 951 rebuilt motor is have a serious hesitation/bog issue. If the outside temperature climbs over 80 degrees, after 15 minutes or so on the track i feel/sense the slightest of hesitation while on throttle. Within 1 or 2 laps the slight hesitation becomes a much larger hesitation and then eventually turns into a big bog. If I keep driving the car, the bog turns into no forward momentum with constant backfire thru the exhaust. If I shut the car down for a few minutes, the car starts normally but within a few minutes the cycle begins again. I have tried to replicate the problem on the street and on the dyno to no avail. So far I have replaced the MAF, Fuel Filter, Fuel Pump, DME Computer, Coil, checked the gas tank for sludge, and tried stock chips (currently running MAX951 from Lindsey). My wires are new with no visible sign of where, the ground cable has been pulled, cleaned and reseated. Nothing has worked. The rebuild was last year and the car ran flawlessly for several track days late last year and into this year. This problem started in late May. Any suggestions? Thanks in advance,

BGA9512

It sounds like a fuel starvation problem to me. When it happens again, try slowing down quite a bit, letting the fuel pressure build back up and see if the problem goes away for a short time. Also see if the problem occurs with a full fuel tank or just when it is running low, which may indicate a tank breather problem. Intermittent problems like this can be a bear to track down, but you just haven't found the exact trigger yet. I guess you are running low on fuel pressure because of the higher demand on the track. The high temp may or may not be a factor. You will probably need to get a gauge to test the fuel pressure at the fuel rail

rshanholtz

Thanks, I have experienced this problem with all levels of fuel. Farnbacher Loles has been trying to track this problem down and has measured the fuel pressure as well. All checks out normal on the dyno. Once the problem starts, I have tried crawing, coasting, flooring the trottle...I have tried it all. I also had the tank pulled and inspected.

BGA9512

If it doesn't "recover" when you drive slowly, then it is probably not caused by a weak pump, partially blocked filter or line. Have you tried replacing the DME relay? It may be getting hot and open the connection to the pump. Switching off and restarting may be enough to recycle for a while until it reheats again. Anyway it's any easy and cheap test.

rshanholtz

Thanks again, the DME relay was one of the first things we replaced. I stuck with all of the cheap things first. Almost everything related to the engine is new. UGH

SamGrant951

Sounds similar to my ongoing issue. spare/freshened motor installed, started having fuel press issues. Replaced the regulator and it ran fine on the street and dyno with lots of boost, took it to a track event in 08 and it started hesitating / going lean on low boost. Pull off track, pop the hood..mess around with it and take it on the street - totally fine. Take it back on track and it happens again. Repeat a few times.

I then replaced fuel pump, fuel filter, regulator again, tested/replaced the fuel damper, TPS, tank screen, flushed the tank, dme relay, checked grounds, pressure tested and took it back to the track with more success (ran a whole day but finally started dropping pressure in the last session) drove fine home under its own power.

Since then Ive replaced the alternator as it failed a shops load test and I havent had a chance to take it back to the track. I also drilled a hole in a spare gas cap just to eliminate the evap system from the possible problems when I try again. Runs totally fine on the street. We will see if it acts up again sometime this year.

Interested to see what yours ends up being!

Rock

I had this happen to the red car a week ago. Not saying its this same problem but maybe food for thought.

The car has 1986 gear, but I would take it out for a drive and it would misfire and spit huge puffs of smoke out of the exhaust. Then it would eventually turn into the car stalling out and dying, like the car hit a big brick wall.

Turns out one of the injector plugs had frayed wires and were contacting. Which somehow was making the whole car die. If the car sat for a matter of minutes it would start right up and run fine.

ehall

First check your plugs. How new is the fuel filter? How are the plug wires, distributer cap, and rotor? Have you checked the speed and reference sensores? Have you checked the conditon of the ref wires ans well as the injector harness wires?

Quickbeam

I had similar symptoms. Turned out to be a faulty DME chip. Worked fine while cold but got progressively worse as the DME "warmed up" until the car became completely unresponsive. It would even run without the MAF plugged in. Did you change chips just before this started? Do you have a different chip you can try?

rshanholtz

Thanks all. All wiring harnesses are new, I have swapped chips, replaced DME computer, new fuel filter, fuel pump, new plug wires, engine rebuilt last year with all new plugs, cap, rotors, injectors, checked gas tank, MAF, coil, cleaned and checked ground cable, DME relay is new, TPS is new...

ehall

pull the plugs and take a look at them.

rshanholtz

Thanks ehall, what are you thinking and what should I be looking for?

ehall

I'm thinking that checking the plugs is a very easy and free way to see if you have internal issues. Look for too much oil. Look for plug or plugs that look steam cleaned. Look for scorched plugs, etc.
Sometimes plugs can tell a lot about an engine.
Plus, I just went through something similar. I changed and gapped the plugs and fuel filter and the hestitation is gone.
You also need to run both your power leads and your grounds. They get hot and wear out, or in the case of grounds, they corrode. Hot leads can arc and cause similar problems. Grounds can do any number of screwy things.

ehall

BTW use the factory plug wrench, in the little tool kit, if you have one. It's a good tool for the job, and it makes it easy to get to the plug as well as replce them, without over torquing.

Vincent C.

hey roy...did you try switching the klr box, of course you have to watch the boost if you don't swap the chip out....keep us informed....vinnie