Hey guys so I've been trying to diagnose this intermittent miss my 1994 Z28 has been having that's become worse since I bought it last month. I daily drove it for over a week with no issues and great power until it started losing power across the powerband then became too unreliable to continue driving. I thought it was an ignition coil and wire problem (that turned out to be just one issue) and I fixed that. But after a bunch of troubleshooting and little issues fixed here and there, the car idled well enough again and I test drove it around my neighborhood (maybe a 15 mile drive) and it did good enough that I wanted to drive it to town yesterday.

Well it drove fine on the way, but when I tried driving home from the restaurant my fiancée and I went to, it barely cranked, stumbled, and missed badly until it finally wouldn't crank anymore and we had to have it towed home. (Second time it's left me stranded. First time was the coil lol).

So tonight I found my old fuel pressure tester and hooked it up. When I turned the key, nothing happened on the gauge but I did hear the fuel pump prime. So I turned the key off and on 5 more times and the needle reached 5psi but that was it. Wouldn't fire when I tried cranking it. So I sprayed starting fluid in the throttle body and it fired right up, but ran terribly, and the FP tester needle never reached past 5psi before I shut the engine off after running for 20 seconds or so.

Therefore I guess I have a fuel problem. I've read about people removing some vacuum line to get a different reading but I'm not sure exactly what they're referring to. So what I would like to know, is there another way I need to test the pressure before assuming it's the fuel pump? Also, how would I be able to tell if it's the fuel pressure regulator that's causing my problems? I do plan on changing the fuel filter to see if that helps anything before diving into a pump replacement.

Any advice would greatly be appreciated. I hate that I can't even enjoy driving this car right after buying it from someone that used it as a daily driver and never had a problem out of it. Thanks for reading

 

posted this answer on the other forum

The vacuum line is the one going to the fuel pressure regulator on the rear of the fuel rail where the schrader valve is. There is a "L" shaped rubber nipple. You pull it off and plug it to check fuel pressure with the engine running. KOEO method you don't need to pull it off

...but in your case given your gauge, assume it is accurate, shows 5 psi your fuel pump likely died. You can take a rubber mallet and pound the bottom of your fuel tank under the area where the pump is to see if it comes back alive...and if it does it will only be temporary but some have done this to get home if it craps out on the road somewhere. It does not work all the time though

 

Ditto..... posted a link to Shoebox's FPR/Schrader valve/vacuum line photo on ltxtech.com

I would also check for 12 volts at the harness connector above the rear axle.

http://shbox.com/1/fuel_pump_wiring.jpg

http://shbox.com/1/fuel_pump_connector.jpg

 

Since this is a recently purchased car, and if you don't have any maintenance history, you may want to change the fuel filter. It may or may not be causing your issue, but I've seen very similar issues with a clogged filter. It's a hail Mary but worth a shot (cheap and easy). If not, it's always a good idea to replace the fuel filter when the fuel pump is replaced.

Good luck...

 

Generally when fuel pumps fail...its like a light switch that just shuts off with only a stumble running out of gas moment. The "why" often is the pump itself but as noted by others check the wiring connector that plugs into the fuel pump assembly for any signs of melting, broken, frayed wires located around the rear end, top front of gas tank. The fuel filter needs changing regardless if unknown history and "may" be the issue alone and worth a try to do first.

Also pull the trunk mat and see if PO did the "trap door" mod. It would make access to the pump 99% easier and also be a clue someone has replaced it before. If that is the case I would take a VERY close look at the bulkhead wiring connection and condition of the wiring leading up to the engine compartment. Fuel pumps often last the lifetime of the car but like any other car part can fail. repeat failures mean you are buying crap parts or the wiring to them is corrupted somewhere

 

He already found the trap door, per a post on "the other site".

 

Any codes?

 

No CEL as of now. Before finding the fuel problem though, the car had an issue where it would idle perfect on cold starts but as soon as the engine warmed up and the idle dropped, the check engine light would come on along with the cooling fans and then it would start idling awfully and missing badly.

So since I have a 1996 Z28 that isn't drivable, I swapped the ignition control module off of that since I read that that if they're bad, they will cause a miss once warmed up. That didn't help much as the CEL still came on. Then I remembered I had a set of brand new AC Delco o2 sensors for the '96 that I never installed so I went ahead and changed the driver side o2 on the 1994 (but then discovered there is no way I myself can get to the passenger side o2).

But ever since I changed just the driver side o2, the CEL never came back on when the engine warmed up and it no longer had any idling problems once going into closed loop anymore. But then again, I probably only put 20 miles on it after that before the fuel pump gave out.

I currently don't have a way to check for other codes but I do plan on getting an ALDL cable soon along with borrowing a family members laptop

 

you have a OBD2 (96) motor so you can buy one of many OBD 2 scan tools to read codes and they even offer things to check specific to that code. Even just a basic code reader alone is around $20 but the more advanced ones are $100-$200

also various laptop programs and a cable is another way

when your CEL did come on....did you get what that code was??

when a car has low fuel pressure, for example, it will stumble, misfire. That stumbling and misfire can show up as a 0300 code (general misfire) or even cyl specific to the cyls actually misfiring due to low fuel pressure and those would be 0301-0308 (specific to each cyl). Those codes don't tell "why" there is a misfire. In this case using low fuel pressure as a reason. If fuel pressure was good those codes would then indicate more a ignition system problem but you would still need to check several things in the ignition system (opti, coil, ICM, plug wires, etc) to find the "cause" of the "misfire" code. Optis can throw specific hi or low resolution codes that point to its malfunction but misfire codes can be caused by many items

Having tools like a code reader or scan tool, fuel pressure gauge, vacuum gauge and IR temp gun are mandatory to have in your toolbox to diagnose engine problems, IMHO.

 

I was just wondering if you are using an accel coil?I blew one up 10 minutes after I installed it(new),on my old bird.Also,did you check the wire harness connecting to the opti?They get burnt.Also,are you loosing any coolant?When my (bird)'s water pump was going south it would drip onto the opti made the engine run real ruff after it was warmed up.Had it towed and new waterpump installed and was lucky the opti didn't burn up.I also lost my map sensor and I thought I blew up the engine.It was banging around something fierce.(mass air sensor I believe on your car).But I would borrow a code reader first.You said any comments,so I posted this.Hope this helps.Check with autozone,o'reilys ,ect for free loan program for guages and tools.My02.

 

The car I'm having the fuel problem with is my 1994 Z28. I also have a 1996 Z28 that is undriveble for other reasons but is otherwise a complete car which is why I installed the '96 ICM onto the '94. But the 1994 is an OBDI computer so no one around me has an OBDI scanner and I've read that a lot of the OBDI scanners available are hit or miss with these cars for some reason.

And also, the coil that was on it from the previous owner was an MSD. It wasnt producing enough spark apparently so when I put a new AC Delco coil on it, it fired right up. So that's still what's on it currently. But it does have Accel Extreme 9000 plug wires on it

 

ah my bad...the post is about the 94 OBD1.

I have a OBD2 car so not familiar with specifics on pulling OBD1 codes other than reading use a paper clip jumping pins A&B on the OBD port with KOEO and counting the # of CEL flashes. I believe the first series of flashes is 12 (one long flash followed by two short ones) which is telling you the car is in diagnostic mode. The following flashes would be the code itself (3 short followed by 5 short would be code 35 for example)

Ideally someone more familiar than me can post how to retrieve codes to help your diagnostics

 

Go to the beginning of this same LTI forum and look for the software SCAN94/95 Sticky that is located there.

It is a Free download from this website provided by Gary Doug. Read through some of the info in it so you understand how to connect and use it. After running it you can upload it here for evaluation by Super Moderator Injuneer.

It has helped me as well as many others.

 

Oh yeah I plan on downloading the scan 94/95. I just have to buy an ALDL cable first and borrow a family members laptop as I don't have one.

And unfortunately the paper clip method of checking codes only works on the '93 Camaro so I really don't have that many options lol

but as mentioned earlier, I found that someone has already done the fuel pump mod so I'm going to open it up today when i get off work and check the wiring