Bones232

Wonderin’ if anyone can set me straight? I’m having an issue at lower engine rpm. When the engine speed is below 1025rpm (and Map is between 30-75) it becomes very difficult to properly fuel the engine. Usually the wb and nbo2 go way lean (18 to 20 afr in OL). This problem is most prevalent when the motor is fully warmed up (180-195degF CTS / 95-120deg IAT).

Originally I suspected that my SPW was on the low side (.8-1.2 msec) causing erratic fueling. Just recently I installed an areomotive VAFPR in hopes of increasing my SPW at lower engine speeds. Low and behold, SPW increased (3.5+ msec) and the problem is still present.

Next I began to increase the VE in the low engine speed areas. No change. When I maxed out the VE table in those areas, I began to raise my BPC just to prove that I could drown the motor in the problem areas. I couldn’t.

I wonder if air velocity through the engine at the low rpm could cause these problems. Is it possible to have a low enough air velocity through the intake and heads that the air cannot carry the fuel to the cylinders? As noted by numerous posts, I usually run little to no IAC steps when hot.

I’ve also read lots about cam overlap at idle giving false wb and nbo2 readings. Even though my cam has a 110 lsa, the duration is tinny tiny. I wouldn’t think it to be an issue. Even so, when the motor reads lean it runs rough and skips. The exhaust doesn’t smell of raw fuel either.

What is interesting is that the engine operation is night and day difference between a hot and cold. When the engine is cold (after startup fuel and SA are decayed out) I can run down in the above rpm and map ranges without any trouble. Currently the cold settings are very similar to my hot setup (OL: AFR vs CTS & VAC, SA vs CTS, etc.). I wouldn’t expect to see much difference in fuel delivery through out the temperature ranges. I was half expecting to see the mixture richen up the hotter the engine gets. That however doesn’t seem to be the case. The biggest differences between the hot and cold settings are the IAC and AE values. AE is not active when the engine goes lean.

Anybody have any insight?

RBob

iTrader: (1)

How does the engine run during this time? Is it chugging or gargling? Or, does it hesitate, and possibly surge?

Misfires will cause a lean AFR to be reported. It sounds like it may be an ignition issue. As things heat up the coil, pick up coil, or ignition module may be getting intermittent.

RBob.

Bones232

The car feels like its on the verge of a back fire through the intake (lean condition i suspect). It looses all power. When the go pedal is depressed (no AE) nothing happens as far as the lean condition goes anyways. No surge is apparant. The motor seems content with hangin' out on the lean side.

Attached is a datalog of one instance where this happened. I'm running OL during this log. Also attached is a similar datalog (from the same drive) with a slightly higher rpm.

Just so I'm clear,
The coil is directly behind the TBI and before the distributor (plug wire goes to center distributor post on the cap).
The ignition module resides under the distributor cap.
Where is the pick up coil?

dimented24x7

iTrader: (2)

I would say that if it doesnt keel over, detonate, or backfire out the intake then the fueling is OK. It definatly could be another issue manifesting itself as a false lean-out.

I have similar issues. With mine, it goes to 20:1+ at high MAPs. The condition persists as long as I hold the engine in that area and it eventually breaks up. It takes alot of fuel to resolve teh problem, and its definatly not an issue with the fueling itself. The fuel delivery is within reason, and the grams per second airflow is even higher then it should be. It seems like a possible issue with the cam or poor fuel distribution. I just installed an isky camshaft in place of the RV cam (same as yours). Should be interesting to see if the problem resolves with a new cam. I personally think its either overlap or poor velocity within the intake.

Bones232

When your engine is cold does it run better in those areas? Mine seems to work well in the problem areas with a cold engine.

Side note; what is an isky cam? Did you have to work your heads to accept a higher lift cam? Reason i ask is i'm contemplating a new cam also. Not sure if I want to try a 'ghetto grind' for higher lift or work the heads the traditional way. She's just a street runner. Not a whole lot of abuse compared to a track car.

Hey....I'm finally a member again....last time i became a member they change the criteria from 35posts? to 100....Junior again....

dimented24x7

iTrader: (2)

Most of the time its OK in the summer, but the winter in sub-zero temperatures seems to be the worst. Its ok under load due to the richer AFRs but if the idle speed is allowed to drop too low or I let the clutch out too fast during the first 10 minutes or so of operation, it gets into a funk and wants to stall. The wideband shows very lean, likely due to the fuel coming out of suspention. So long as the RPMs are kept over 1200 during high MAP, its fine. Lower, and it wants to stall out.

The isky cam is another stone-age grind, 208/216 .435/.445. The largest dual pattern I can confidently stick under the stock heads w/o mods. I wanted to go larger, but that would require not only head mods but that I get rid of the 2.77's out back, which might not be easy (or cheap) with a 9-bolt. TKO500 + highway gears = crappy gearspread. With a large cam, anything other then 1st and 2nd would be bog city with the XE268 that I want to run.

Bones232

I've often wondered how large overlap engine acts that requires the idle/gearing/stall to be raised. Maybe lack of velocity and a lean afr is it???.... for the time being i'll go over my ignition system again and fiddle with the tune more.

I'm also dreaming of the XE268 cam. Seems like a good fit for the vortecs (moderate lift and decent duration).

Bones232

Another thing i've been staring at is the fuel pressure. Attached is a fuel pressure comparison between a cold engine and a warm one. Notice the erratic behavior when the engine is warm.

Is this normal behavior of a pressure transducer as ambient temperatures increase? I realize mechanical pressure guages are sensative to ambient temperatures skewing readings.

The mechanical guage on the regulator does not oscillate as the transducer does.

RBob

iTrader: (1)

I didn't realize there was a fuel pressure transducer installed. Looking at the first two log pic's you posted the fuel pressure is all over the place. In the lean log it is as low as 2 -3 psi, and as high as 20 psi. Even with the VRFPR it shouldn't be dropping below 10 psi. And even that will be tough to get as low as. (25 psi fuel pressure minus 14.7 psi atmospheric is 10.3 psi).

I have found that the transducers will oscillate more then a mechanical gauge. They just react faster. However, it is usually within a few psi as it picks up the pulses from the positive displacement pump

RBob.

Ronny

iTrader: (1)

I have experienced what i think(?) is fuel falling out of suspension occasionally when the ambient temps are about 75F and engine is at 185F yet the manifold has not yet been fully heat soaked. Occurs after I leave office and make a L turn at busy intersection about 8 blocks away on a cold start. often at the wrong time engine will bog from a stop and wait putting my ride in harms way. manual trans. drive another couple miles and do same no issue. as the butterfly's open the MAP goes way up-fuel falls out-bog. when mani heated no issue.

EDIT I was typing as RBOB was?
Is the "transducer" a electronic fuel pressure gauge(in passanger compartment)?

Is that not a huge fuel pump delivering more than enough gas? could the pump be somehow becoming intermittant?

454 TBI-80 lbs-20 lbs FP-holley projection intake-170cc intake port-224*230 @.05 114LSA

Bones232

Sorry if I didn't catch this, but are you saying the pressure fluctuations are a combonation of the ambient heat and the ability of the pressure trans to see the pressure pulsation created by the fuel pump vanes? Or should I be looking for a physical issue with my fuel system?

The transducer is a device that converts mechanical energy into electrical energy the ECM/electric guage can measure. It is mounted in place of my original mechanical pressure guage on the fuel supply line. The transducer is wired into channel 1 of my EBL.

Ronny

iTrader: (1)

A while back I saw a post or two on the 255 Walbro fuel pump. Seems 255 may have been on the edge or over as far as the return line not being sufficiently large. I wonder if that could have any effect on fuel pressure not being consistent? Just a guess.

during the event in log what is the duty cycle look like?

do you see the PW flipping synch-asynch back and forth in log?

at idle I am synch only but maybe you need to be synch at idle and up to
1200 RPMS. possibly the ECU is interrupting the injector pulse due to feedback from NB02 sensor.

RBob

iTrader: (1)

The graphs that you posted, the cold one, there is some oscillation in the fuel pressure. A few psi which is normal. This is caused by the pump itself.

The original log files, the Fpsi column is the fuel pressure. Note how it fluctuates. With or without a VRFPR it should hold steady at a steady MAP. With a VRFPR you can take the difference between BARO and MAP convert it to psi, and subtract it from the nominal fuel pressure.

Say at 40 MAP and 102 BARO (BRO), 102 - 40 = 62 KPa vacuum.

Convert the 62 KPa to psi: 62 / 6.895 = 9 psi

With a nominal 25 psi: 25 - 9 = 16 psi. (I'm guessing the nominal is 25 psi from your graph).

The majority of the lean log is at 40 - 41 MAP Kpa, the Fpsi column should be at 16 psi.

RBob.

Bones232

So what would you say is causing the erratic pressure swings (and could this be my problem)?

I'm not convinced its a return line size issue. Mostly because the fuel press grouping when the engine is cold is tight and consistant. Hot engine, not so much. I also had this problem when the original TBI pump was installed and pressure jacked.

Pump and filter are only a couple of years old.

I've also noticed excess pressure in my fuel tank. A while back i was fiddling under the hood when i saw my mech fuel gauge surging (2-3psi). For kicks i unscrewed the gas cap to relieve the pressure in the tank and the fuel pressure fluctuation ceased. However, while driving the car it still acted as decribed above even with the cap unscrewed.

p.s. DC was around 11-14% and I'm running sync fueling all around.

RBob

iTrader: (1)

If the return line was too small the fuel pressure would be too high. The vent valve may be bad, that will cause excess pressure in the tank, although the CCP system should open before the tank valve does.

CCP pressure control valve opens at 5 KPa (and higher), the tank vent opens at 5.5 - 7.6 KPa. The tank vent also opens in the opposite direction to allow air into the tank. This is at .88 - 1.9 " Hg.

As for the fuel pressure, this is an issue. Falling as low as 2 psi will cause the engine to lean out. It may be that the pump is going bad, or the hose between the pump and output tube has ruptured. It may also be that the fuel filter is installed backwards.

RBob.

Bones232

Is a stock style fuel filter adequate for this kind of flow/pressure? Are there better filters to use?

I'm not looking forward to dropping the tank to check the sender (sounds like a winter project). I'm fairly confident the pump is ok. I recall having similar issues with the old TBI pump. I don't recall the condition of the intermediate hose between the pump and outlet tube though. I reused it when changing pumps. Maybe the connections leak.

When i shut the engine down, after couple of minutes i usually hear a noise coming from the muffler/tank area. It sounds like someone lightly tapping a half full drum of water with a stick. Its subtle but noticeable.

I've often suspected that the tank is under a slight vacuum which pulls the tank walls in slightly while the engine is running. And with the motor off, air leaks back in the tank releasing the vacuum allowing the tank walls to return to their atmospheric position (causing the noise). However when opening the fuel cap shortly there after it feels like the tank is under pressure. I can see, and smell, the vapors release.

The vent tube you speak of is one of the lines clustered on the sending unit, correct? If so, its been on my list to replace. It doesn't look too healthy.

RBob

iTrader: (1)

I use the stock replacement fuel filters from WIX (CarQuest) with no issues. The arrow on the filter should point toward the middle of the vehicle.

The vent is a plastic mushroom looking device stuck into a bracket above the rear axle housing, drivers side. Is the CCP system still intact? Maybe the CCP line and return line are reversed at the tank?

Can try driving it a bit with the gas cap loose. If the issues go away and the fuel pressure holds, then we know what the problem is pointing too.

RBob.

Bones232

Curious....if they were reversed wouldn't i suck fuel into the CCP canister when the sov is energized?

RBob

iTrader: (1)

The tank pressure will do it. The return line in the tank being lower then the fuel level. And once pressure builds it will push the fuel into the canister. The canister has an air vent so it may or may not pull fuel from the tank with the solenoid open.

If the canister isn't full of fuel, then it probably isn't an issue.

Apparently reversing these lines is easy. More then just a few here on TGO have had this issue. Some day I'll find out for myself when I do the FP swap.

RBob.

Bones232

You might be on to something here. I haven't noticed any fuel coming out of the CCP canister, however I have had a strange fuel smell lingering in the garage after shut down. seemed to be coming from under the hood. It seemed to start around the time i swapped the FP and replaced my rubber hoses with -6an ss braided. I'll have to double check the connections. I'm out of town so it won't be until sunday morning/evening.

By the By, does anyone know what the connection layout is for the sending unit? Maybe right to left as you're looking from the rear of the car. IIRC, there should be 4 connections (supply, return, vent & ccp).

Bones232

sadly all lines appear to be hooked up properly. The CCP and return lines are different sizes. The vent line and return line however are the same size. I put a vacuum pump on the vent line in attempt to siphon fuel out of the tank. If it was below the fuel line (as the return line is), I should have gotten fuel out of the tube (but didn't).

I also checked the vent valve for operation. It appears to vent both under vacuum and pressure. Seems ok.

Next I checked the operation of the CCP control valve. When vacuum was applied to the downstream side of the valve flow was apparent around 2.5-5 in. hg. When vacuum was applied to the upstream side of the valve, vacuum was maintained (no bleed back). According to the shop manual, the valve appears to be working properly.

Looks like i'll be dropping the tank after all. I've been reading about the pulsation damper and its tendancy to fail and leak. I've also read that it seems to be acceptable to replace it with a straight length of tube. The posts I found were several years old. Is this still a common practice?