Hi all,

Thanks for the wealth of knowledge on this site, I've been using it over the past two weeks as I bring my new to me IROC back to glory!

Bought the car a couple weeks back, fully restored exterior and interior, stock but running well 305 TPI, five speed car.

Had an issue where at cruising throttle speeds, the car would buck unless given more throttle. Previous owner told me that he had hooked up a fuel pressure gauge and gone for a drive - noticing that the fuel pressure dipped every time this bucking occurred. He replaced the fuel pressure regulator, but presumed that it might be a weak pump.

I bit the bullet last weekend and installed a walbro 255 and racetronix hotwire kit, but the issue remains albeit less so.

This weekend, I took off and hot tanked the intake (upper and lower) did new gaskets on everything, cleaned up the throttle body, new IAC and egr (both were seized). I also followed the guides for setting min air and timing. Now I unfortunately have an idle issue, where I can set it drive around and it will eventually creep up to 1500 RPM. Checked all vac lines, re-set timing gone through all the paces that I can think of.

Now I'm just waiting for an ALDL cable from moates so that I can properly datalog.

Things I'm considering: MAF sensor? O2 sensors? Fuel Injectors...


Any help with this issue would be much appreciated.

 

I would check injectors for sure as well as coolant temp sensor.

 

Yeah, the coolant sensor thing passed through my mind as well - which one is the one for the PCM and which is for the gauge?

 

https://www.thirdgen.org/forums/tpi/...nsor-part.html

 

OK so I got my Moates aldl cable tonight and hooked up TunerPro RT.

The software is very comprehensive and it took me a bit to get what I wanted all set up.

I took the car for a quick run while logging and found that my target air/fuel ratio is constantly in the red, as well my O2 sensor was consistently out of target as well. I happened to have picked up an AC delco replacement O2 so I put it in this evening. Now my O2 readings are consistently in the black, but occasionally dip into the red, but the target AFR is still constantly in the red.

Any suggestions?? I have a south bay adjustable fpr waiting to be installed- is it possible that after installing the walbro pump and racetronix hotwire kit that my stock fpr is insufficiently dampening the pressure?

I will continue the quest for glory...

 

Do static and dynamic fuel pressure tests.

Static = motor off, key on, and check leakdown time too. Post time of pressure loss.

Dynamic = 1) motor on, vac line to FPR off and capped. 2) vacuum line ON, rev it a little, looking for pressure boost, in response to vacuum increase. 3) WOT, with guage taped to windshield.

Post results.

 

Ok, did a quick set of tests this evening:

Static pressure: 15 psi

Dynamic: vacuum connected = 34psi, revs cause decrease to 30psi

Vac disconnected=43.5psi


Will post leakdown and WOT pressure in a bit when I can tape the gauge and have a bit more time!

 

Does anyone have a .PDF version of the shop manual?

 

Good job so far.

You might be able to get one on eBay for cheap. BE CAREFUL of what you get. It hsould be about 1,000 pages worth', give or take.

15 psi static FP is no good. It is your STARTING point diagnostics.

Disconnect the FPR, cap the line - not by 'pinching'. Try clamping on a vacuum port cap. COLD motor, in case the cap leaks. Do the test STATIC test again.

Nothing else right now. Period.

 

Okay disconnected vacuum line to FPR and capped the line. primed the fuel system: 16 psi from a single prime.

started car with the vac disconnected: 43.5 psi

I knew that the FPR had been recently replaced by the previous owner, but to be thorough I removed it. It seemed when I was carefully removing it from the diaphragm that the rubber was pinched a bit on one side. I carefully installed an adjustable regulator being sure not to pinch any of the diaphragm. adjusted to 43.5 psi without vacuum connected.

Now when i blip the throttle, there is a corresponding increase in pressure, rather than a decrease. took the car for a drive- noticeably less hesitation from initial throttle tip-in.

Now for the bad: I also paid attention to the bleed down. from the key-on prime of 16 psi, the pressure bled down to nothing within about 30 seconds to a minute.

 

All that tells you is that the low pressure is NOT from a ruptured FPR bladder.

It does NOT tell you that your low pressure COULD be leaking injectors, or COULD be a weak spring in the FPR.

You need now to eliminate possible weak FPR spring, by capping the line BEFORE the FPR - best by removing it and capping the line - not pinching.

Then you can tell that the low pressure is SOMEwhere between the pump and the injectors.


ed.:
The later part that you posted, that pressure increased with rev's is good. That tells you that vac supply for dynamic FP test is doing it's job. Good job on that.

But STATIC FP diags is FIRST, before dynamic FP test. You HAVE to find out why only 16 static FP.

 

I was thinking about this when I woke up. My fuel pump does not run constantly when I turn the key to the on position. I don't know if this is because of the hotwire kit that powers the pump from the alternator? I also know that the FPR spring is only a year old. When I pulled the FPR spring out to check the components, it was stiff as can be.


I will cap the line to the FPR just to be thorough, but I'm nearly 100% that I have injector issues. Why else would the pressure go to 0 only moments after turning the key off?

 

Sounds like injector issues.

 

That's the way it is supposed to be, as it pushes fuel pressure to 43.5 (about), and then shuts off. WITH THE MOTOR OFF.

You will find out after you do as I directed.

Post results, with leakdown time.

 

Not to state the obvious but have your checked for engine codes ?

 

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Gauge sender is a 1 wire , spade terminal unit located in the head

 

Yep,

I bought an aldl cable and did a bit of logging with tuner pro. The only issue that showed was an out of spec fuel mixture.

I believe I have isolated the issue: I found that high flow pumps typically don't have check valves, and in order to achieve 40 psi from key on, it is normal to have to modify the duration of the prime cycle.

I did cap the fpr line, and nothing changed. I installed a fresh set of injectors from south bay. Initial test drive was good. Better idle, no lurching.

Will update when I have a chance to datalog!