If You Think Your Good At Diagnostics..
 

Hi everyone-

Hopefully someone here can solve my problems. I have a crate ZZ4/tpi with a Hotcam and 1:6 rollers. I have had this engine in the car since 96’ it has been chipped since then as well. Always ran perfect with a slight idle swing of 100 rpm. No codes have been set.

Now, the car has a surge at any speed. It feels like the air cond. Is being turned off then on again. Also, I have a vacuum issue: with my gauge attached, the vacuum has a steady slow swing from a high of 14 inches to a motor stumbling 5 inches at idle. My fuel pressure swings in corralation with the vacuum swing at idle. The rpm’s are cycling up and down at idle from a high of 1000rpm to a low of 500 rpm.

Under power, the fuel pressure stays steady and the rpm’s stay steady,but the surge is there..

The following diagnostics have been done:

Disc. Est while running…no change.
Compression check…all cyl. at 210 lbs.
Rockers re-adjusted by a shop.
Checked for vacuum leak around outside of all intake parts using spray method..no leaks.
New parts: tps,iac,o2,cap,rotor,mat,plugs,fuel filter.

Swapped out these parts with known good parts with no change: ecm, dist.

I have had two different sets of injectors all 22lbs. Swapped in and out….no change.

My afpr holds pressure after key off…no quick leakdown.

Took off base intake and re-sealed w/new gaskets twice…no change.

Un-hooked egr vacuum under plen and plugged vac source…no change.
Un-hooked canister vac/plugged source…no change.

Checked charging syst..checked fine.
Checked all grounds…all there.
Checked ohm resist on injectors….all around 16-19.

Unhooked maf during running…no change to the swinging vac/rpm/f.p.

Car has these sympt. In open and closed loop.

Loosing electrical power if the fuel pump pressure is changing pressure at idle.
Check to see if your voltage is dropping in those circumstances.
Could be a corroding connection somewhere in the power supply wire to the pump/relays.

That doesn't necessarily have to be an electrical problem. His fuel pressure will be swinging as engine vacuum fluctuates, even at idle. Going from 14 in to 5 in of vacuum is a pretty significant change.

If I were you, I would invest $30 in an ALDL cable (or make one yourself for about $12), download Tuner Pro RT, and borrow a laptop from a friend. Thats going to help you diagnose this, and any other engine problem much better than any other tool you can own.

92 blue-

Hi'

I have the cable/laptop. can't see any weird stuff going on except the block learns are not up they were like 108?

a scanner and data logging haven't helped yet.

though I am not great at using the data log to my advantage.

Throttle position sensor or 02? TPS can act like that.

Does the fuel pressure change with the vac line off it and plugged? What brand AFPR is it?

tps is new, o2 is new.

afpr checks out. no fast leakdown after key off. passes vacuum test.

same symptoms w/two diff. fpr.

Loose throttle body shaft? Loose butterfly?
Maybe some vacuum leaks on the TB itself.
Doesn't seem like something you would normally check but could have effects.

Blown out PCV valve?

Swap the battery for another.

I think that you have multiple problems...as for the surge, you haven't mentioned replacing the EGR valve...I have experienced a surging like you describe from a bad EGR valve, on two different vehicles (a TBI 350 van and a TPI 350 car).

As for the huge idle swings, I think you need to do a more thorough vacuum leak test...unplug/plug all vacuum lines, except those necessary to the engine, ie fuel pressure, map sensor. See if that changes the idle characteristics. Then check for a bad gasket on TB, intake, etc.

I would also check your battery voltage at idle and then at 2500 rpm...just to make sure that your alternator isn't on the way out. Low battery voltage can cause the ECM to do wierd things...

HTH...

I think you've found the problem already with the oddball vacuum readings, you just need to pinpoint where the problem is. I understand how much of a PITA that is. The thing with the fuel pressure is also kind of weird. I remember when I was troubleshooting my idle/stall problem (which ended up being bad IAC and bad TPS) my fuel pressure was steady even with the RPM's going up and down. I think you should focus on those two things, trying to find a vacuum leak and testing all aspects of the fuel system (start back at the pump measuring voltages and work forward)

check the distributor, module and pick assembly. If you have a known good spare module, swap it in. Also, check the condition of the connector from the module to the pick-up. they are well known to corrode and basically melt and cause weird ignition problems, which could generate symptoms of weird idling/surging.

just throwing that out there......

Isn't a 108 block learn the bottom of the programming's A/F adjustment range? If so, I'd think the ECM beleives there is a seriously out of whack A/F ratio and it's trying to compensate (and failing to do so, or it's being fed bad data and reacting to it).

Agreed.

Measure the O2 sensor's voltage. If it's consistently low, then it's reading lean and the ECU shouldn't be removing fuel, which it is.

Pinch off the hoses to the canister purge and the EGR.

If you want another piece to change to see "what if," try the EGR valve. Also, do the ignition module.

Try opening the throttle blades some, disconnect the IAC, then see how bad the surging is. Then at the same time, disconnect the EST Bypass, and put a timing light on it.

It'd be a lot easier with a scan tool on there (for me at least). Could see if any of the sensors are giving bad data easily, then see what the ECU is thinking.

vacuum loss at the tb shaft and any intake gaskets has been ruled out. scan tool shows everything reading normal except the block learns at 108.

new:
afpr
inj.
module
tps
iac

the egr has been disconnected. and no change.

have pinched off the hoses at the canister..no change.

o2 readings per scan tool look good.


but when I pull spark plugs, I don't smell any fuel..shouldn't I?

if you unplug the IAC and start the car, does it still surge?

surge yes w/iac disc. surge w/maf disc. surge in open/closed loop.

A plug wire or two arcing??

What's left?

What about if you disconnect the ECU to reset BLM. Then disconnect the O2 sensor. Then drive.

disc. the o2...no change.

swapped ecm's....no change.

You have done so much that I might have missed it, but have you done a compression test, cylinder leakdown test. Flucuating vac readings could be caused by problems with the valves.

Are you running an underdrive pulley?

compression check as all cyl. at 210.

new roller rockrs were just adj. by a shop.

I doubt. it's valvetrain.

With everthing you've checked I think I would start looking at the valvetrain. Broken valve spring, coil bind, bent push rod, who knows. I would pull both valve covers and unhook the coil. I would then stick a dial indicator on each valve or roller rocker as you turn the motor over. It all started after the heads went on right? Thats where I would start to look now.

Possible intake restriction on the fuel pump?
Could have pressure but it could be cavitating causing air bubbles in the lines.

You mean, he could have one of these?

http://www.classy-touch.com/files/FieroPump.jpg

Something like that;)
disconnect the fuel return line and run it into a bucket.
that will keep the pressure on the rails and should show good clean volume. If it comes out all bubbly that could be the case.
Be careful not to confuse shearing of fluid with entrained air from cavitation. Hold the hose under the fluid so you can see the actual flow of bubbles rise to the top if it looks that way.

that vacuum is horrible. what type of cam did you put in the car. I had a buddy that put a huge non computer cam in the car and I would not run at all no matter what he did. Are all your vaccuum lines new I mean all of them a small pinhole in one of them could cause a world of hurt. bring a screw driver flat head some plyers and a razor blade to the parts store. I know this might sound getto but the way you car runs is even more getto. take the vaccum lines of one at a time and have the guy behind the counter get you new ones. everyone should do this it costs under 20 dollars looks good and gives you piece of mind.

I agree with jamon8. You ether have vacum leak or a cam that is not proper fit for this application. But before you do anything, check all vacum lines. Be sure to check vacum lines running to the cruse control etc.

no, no, no.

the motor is a zz4 crate with a lt1 hotcam. a known combo.

15 inches of vacuum is just fine if it would stay there.

I am going to put in a new fuel pump just because.

your valves are staying open for too long try putiing the 1.5 rockers back on

I'm betting on a messed up EGR valve (broken on the inside), or ignition module. If the fuel pump were having problems, you'd be able to see the pressure drop then the RPM drop, in that order.

Silly...........

the valves ARE NOT staying open too long. the 1:6 rollers are fine.


the fuel pressure and rpm's do swing together.


I am going with a new pump anyway.

I second an egr valve. I bet the diafram leaks.

is there a way to test the egr valve? on the car or off?

If you take off the vac. line to the EGR and plug it and the idle gets better it's a bad EGR. It was causing a vac leak or allowing EGR in at idle which it shouldn't. The latter could be caused by the valve sticking open. If you take it off and use a hand vac pump on it to see if it opening and closing completely.

thanks. I will be checking the egr.

yO David, you need to jump on the tuning bandwagon man.
I disabled the EGR within minutes using the software.
I'm sure some in depth datalogging might help to narrow it down.

A very politically correct answer. Nicely done. "the fuel pressure and rpm's do swing together". Are we talking about an engine problem here?!

Get a can of STP THROTTLE BODY AND AIR INTAKE CLEANER and spray around all the intake gaskets, vaccum hoses, egr valve, and vacuum canister and anywhere else you might have a leak. I am a GM mechanic and this is how we eliminate vaccum problems. Also Im not sure what the voltage for TPI is off hand but check your coolant tempurature sensor under the TB this is the sensor that controls some of the rich and lean perameters for the drivability in the ECM. Let me know what the static resistance is on Coolant temp sensor and I will look up the proper reading later on.

We have done the spray around method for finding leaks. none found. even was able to reach the base of the egr under the plen..and nothing.


the coolant switch...I got a new one just didn't put it in yet...what the heck might as well.

sounds like you are in the same boat as me....I have jsut about given up. Did you have any luck>? I'm to the point where i am gettign ready to part the car out. and I never thought i'd say it, i've had it for 10 years.
My Car is not throwing any codes,...so I don't think it's a MAF. plus i disconnected it and it doesn't seem to run any different. I took apart the timing cover today to see if the cam to crank gears were out of sink, but they are not. Just curious, what if the crank gear was installed incorrectly on the A or R? It looks like it was on at 0, but would it even run if it wasn't. The car sounds like it's running out of valave timing. But again, i check the cam gear position and it's correct. I guess it could be a valve hanging open, or a sensor, or an intake leak or a million other thiings....I don't know what to do! I don't want to be just changing sensors liek crazy, they are expensive, plus i would think i would get a code. please help! thanks

Did you say you disconected the MAF sensor and it didnt make a difference. If thats true you probably have a maf problem.

i thoiught it would run in "limp mode" and run better with the MAF disconnected, if the MAF was bad. thanks

no the thing should start stumbling all over the place. Think you just took away its ability to read of much air the motor is getting. So how does it know how much fuel to add.

With the MAF disconected the ECM will use preprogramed values and input from other sensors to detirmine how much fuel to use. When I have run with the MAF disconected my car runs normally, but it will set a code.

Thats funny cause when my mass air sensor went out it barly ran.

that's what i thought. that's why i unplugged, and nothing changed. it's more erratic running than a MAF it seems....

yeah but was the MAF plugged in still, if you unplugged it, it should have run better than with it plugged in and broken