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Discussion Starter #1
Hi Guys,
I've got a 94 Buick Roadmaster with the LT1 and an issue has recently appeared that has me stumped. It is an intermittent rough start issue that first appeared the other day on a hot start. I'd driven up a long inclined road (around Tiger Woods recent crash site in fact) to pick up some kids from school, parked, turned off the car, waiting 10 minutes for the kids and then tried to start it up. The starter worked fine, but the car didn't catch with the first start attempt. I then tried again and after a longer period of cranking it fired up, but then died. I tried a few more times, but it did the same thing, fire up, run rough...cough cough..sputted..ran long enough that I got to the tailpipes -- no nasty smells or fuel smell. Then died. Tried hitting throttle -- engine wouldn't rev up past idle. Repeated this a few times and then it started. Ran great. No problems, including a 20 minute drive home. Mmm. Started again the next day, I drove it 5 minutes away (no hills this time) and parked it. Came back 45 minutes later, same thing. Cough cough..glugg glugg...sputter die. Tried 3-4 times, eventually it started. Drove home great.

No CELs no MILs -- I even ran into the onboard computer interface via the climate control and looked for faults, nothing. Nothing recorded. I've got the big factory service manual and I'm looking at "Hard Start" section and there are quite a few items to check. ECT, MAP, MAF, TP, Fuel Pump relay, Fuel pressure, injector fuses, plugs, ingition coil, coil fuse, dist vent system, IAC.

Recently replaced items include 02 sensors, EGR valve, EGR solenoid, TPS sensor.

The issue I'm having with following the manual is that the problem isn't there at the moment -- and Im having a hard time replicating the issue (it starts fine right at the moment).

Thoughts? I don't mind throwing a bunch of components at it -- I'm pretty handy with tools, I just don't want to get stranded after putting $500 of parts and a weekend into this thing. Maybe that's the risk of driving a 94 Roadmaster...? 馃榾
 

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You need to find an obd 1 scanner that does running sensor scans. Since these are hard to find, may I suggest the PC based free scan program called "scan9495" It is made for F-body lt1 cars, but pretty sure it will work on your roadmaster lt1.

Go to the following link and download the newest version in the lt1 v8 folder. www.mediafire.com/?0elo4c935uc5s

You will also need dot net framework. It's in a folder on the same page as scan9495.

Finally, you will need a cable to connect car's aldl port to usb port on PC based computer or laptop. Get at
www.obd2allinone.com/aldl.asp

Get the cable with the 12 pin aldl connector. Our member GaryDoug wrote program. If you have questions about it you can ask him. He's more active at www.firebirdnation.com

If you need help understanding sensor data, PM me with your email address. I'll send you the pdf file called "scanner readings and what they mean.
 

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Keep in mind that your car is 27 years old. Wiring, grounds, & bad connector pins can cause havoc. My 90 camaro daily driver has had a rash of connectors that the lock tabs have broken. This causes connector to work loose & bad signals being sent through. The opti wire pigtail is a known troublemaker.

And get a fuel pressure tester. An old worn out fuel pump could be dropping fuel pressure when hot causing trouble. So check pressure when cold. And keep tester connected. Tape gauge to windshield. See if when car is hot and acts up, if pressure falls way down.

Normal fuel pressure at key on prime should be around 43 psi. At idle pressure should be at least in mid 30's psi.

Also keep in mind that opti low & high resolution codes don't light the check engine light. Or service engine soon light, depending on which your car has. Also if both opti low & high res signals are lost, no code will be set.

Only 100% way to check opti low & high res signals is with an oscilloscope. You can get a PC based scope on ebay, amazon, etc, for around $70. The signals are a square wave.

Does your security light come on when key turned to on? Then go off after several seconds? This is normal vats security proceedure. If security light stays on then pcm keeps fuel injectors from opening. From factory it also killed starter from turning engine over. But it's a simple bypass that a previous owner might have done.

Scope will also show 50 hertz fuel enable signal sent from theft deterrent module (TDM) to PCM. It's a square wave also.
 

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Discussion Starter #4
Keep in mind that your car is 27 years old. Wiring, grounds, & bad connector pins can cause havoc. My 90 camaro daily driver has had a rash of connectors that the lock tabs have broken. This causes connector to work loose & bad signals being sent through. The opti wire pigtail is a known troublemaker.

And get a fuel pressure tester. An old worn out fuel pump could be dropping fuel pressure when hot causing trouble. So check pressure when cold. And keep tester connected. Tape gauge to windshield. See if when car is hot and acts up, if pressure falls way down.

Normal fuel pressure at key on prime should be around 43 psi. At idle pressure should be at least in mid 30's psi.

Also keep in mind that opti low & high resolution codes don't light the check engine light. Or service engine soon light, depending on which your car has. Also if both opti low & high res signals are lost, no code will be set.

Only 100% way to check opti low & high res signals is with an oscilloscope. You can get a PC based scope on ebay, amazon, etc, for around $70. The signals are a square wave.

Does your security light come on when key turned to on? Then go off after several seconds? This is normal vats security proceedure. If security light stays on then pcm keeps fuel injectors from opening. From factory it also killed starter from turning engine over. But it's a simple bypass that a previous owner might have done.

Scope will also show 50 hertz fuel enable signal sent from theft deterrent module (TDM) to PCM. It's a square wave also.
cocobolo -- you are the best.

My full back story -- this is the second time I've owned this exact car -- I had it from 2017-2019 and now its come back. I did the chip-key bypass myself back in 2018, so I don't think this is related. With the anti-theft issue, it was a pure "no-turn-over". With this, it is running, but some sensor is so outta wack that it runs rough and horribly. The bummer is that this car has been super reliable and my preferred kid hauler -- so I appreciate the advice with regards to getting a simple code scanner, I can do that (I still have the OBD2 VAG-COM tool from my years of owning a 2001 Volkswagen).

My biggest issue right now is that I can't get it to replicate the problem in my driveway, so I'll need to just throw a laptop with scanner and junk in the car and wait till it strands me in a parking lot somewhere and then do the debugging. Uggg. Cars. Why couldn't I have a hobby like wrist watches or birdwatching.

The only thing I found from following the service manual instructions is that the coolant temperature sensor near the water pump (not the one in the head) is reading lower than ambient temperatures when sitting in the driveway. So the temp sensor in the head was reading 71F and the one by the water pump was reading 14C (57 F). I'm gonna order a new one -- but I can't tell which one is which on RockAuto. 1994 BUICK ROADMASTER 5.7L V8 Temperature Sender / Sensor | RockAuto

There are two different temp sensors, but are they the same part number?
 

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The 2 coolant temp sensors have different part numbers. The wp sensor for PCM has part # 12146312.

The coolant temp sensor for gauge has part # 25037332. GM gets tricky with part numbers different #'s can mean a different part altogether. Or it is sometimes the same part with a different number.

I believe they are 2 different parts. Got numbers from f-body parts book. B- body might or might not use the same parts as f-body. I can't say. If last owner put incorrect part in wp, it would have different reading than correct part.

It is my belief that the 2 different cts have different heat verses resistance readings since a gauge would use different resistance then a PCM.
 

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Discussion Starter #6
Okay. Got the ALDL OBD1 cable (turns out the guys who build/sell them are local to me, saved on shipping!!) and now I've got an old laptop hooked up and am pulling data from the ECU. First thing, the car worked. Then it gave me a crank but not start (i've saved that log and will try to attach here) and then it cranked and started. Oh boy. I can''t see anything in the data to show why it did the crankcrankcrank no start, versus 2 mins later when it started.

Unfortunately, i can't attach xls or bitmaps to this forum.
But here are some links where i can upload them. (edited, the first time something was funky)
Data is here:

Thoughts? Might be fuel pressure because I didn't see anything in the data that jumped out at me.
 

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Have you done a fuel pressure check? Should be around 43 psi @ key on prime.

I haven't looked at the scan data, but does coolant temp that PCM is seeing, match the coolant temp of gauge?

If all else leads nowhere, try using an oscilloscope to read the low & high resolution pulses of opti. Should be nicely shaped square waves.

The opti has a wiring pigtail that runs from opti to passenger side of intake manifold. It goes bad with age. Replacements are sold on ebay.

If you don't have an oscilloscope, a PC based scope is available on ebay, amazon, etc, for around $70. And while your reading the low & high res signals that opti sends out, read the 50 hertz fuel enable signal that theft deterrent module ( TDM ) sends to PCM. It's a nicely formed square wave also.
 

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Discussion Starter #8
Have you done a fuel pressure check? Should be around 43 psi @ key on prime.

I haven't looked at the scan data, but does coolant temp that PCM is seeing, match the coolant temp of gauge?

If all else leads nowhere, try using an oscilloscope to read the low & high resolution pulses of opti. Should be nicely shaped square waves.

If you don't have an oscilloscope, a PC based scope is available on ebay, amazon, etc, for around $70. And while your reading the low & high res signals that opti sends out, read the 50 hertz fuel enable signal that theft deterrent module ( TDM ) sends to PCM. It's a nicely formed square wave also.
I haven't looked at the fuel pressure yet -- that is next. The two coolant temp gauges read pretty different things when i look in the on-board climate control system display -- but I only saw one coolant temp listed in the data from OBD1 device. I just downloaded the data and haven't had much chance to look at it. And the way it didn't start was straight up crank-crank-crank instead of the chuff-chuff-chuffff 400-600rpm running of the other day. Probably related,disease but different in symptoms. gotta get a FP guage now. Let me look through my gauges -- I've certainly got some old boost gauges and a few random industrial gauges around that'll do the trick for fuel pressure, or I could buy something simple.
 

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The coolant temp you see displayed on the data set is the cts in wp that sends to PCM.

If your getting a really low reading when you turn on ignition like 40 degrees below actual coolant temp, engine won't run. If cts is reading way high, pcm will flood engine.

You will need the connector that attaches to schrader valve on fuel rail to get fuel pressure reading with a gauge. You can get a deluxe fuel pressure test kit, with all the fittings for most every car. Get it on ebay, or amazon for around $36.

Once you have the kit, if car runs,idle fuel pressure at idle should be at least 30 psi. All other running conditions, pressure should be at least 40 psi.
 

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If last owner changed the opti, if it's anything besides a GM, Delco, or Delphi, the opti could be your problem. You need an oscilloscope to 100% test low & high res signals from opti.

Also remove icm. Take it to an auto parts store for testing. If bad, get a new one. Don't forget to put heatsink compound on back of icm & it's heat sink. You can get the stuff at stores which sell computer parts. It's the same stuff that's on the back of your PC processor.
 

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Is the lower graph while cranking? And what are you doing with your foot? Throttle position is wild.

Al
 

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Discussion Starter #12
Big Al, Yes I was giving it some throttle on a few of those cranking attempts, but I forget if it was in that actual data set. I can dig into it.

I just received my fuel pressure gauge today, and will do some basic fuel pressure investigation before moving on to more diagnosis and the opti. I've always wanted an oscilloscope for home use (i've used one at engineering school and then work over the years) so this is indeed a great excuse to have the ability to scope things at home.
 

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Discussion Starter #13
Fuel pressure is 25psi on key on. About 32 psi after start and idle. Then approx 38psi when running at higher rpm. New fuel pump and filter time?
 

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It could be something as simple as a clogged fuel filter. Or a fuel pump going bad. But it could be other things like a bad fuel pressure regulator, leaking fuel injectors, a bad fuel pump check ball, or even the pump pick up line leaking.

Simple test you can do is to do key on prime. Then watch fuel pressure. If it leaks down quickly, suspects include pump check ball, pressure regulator, or leaking injectors.

Pressure should take half a hour or more to leak down to 0 psi. If pressure is low @ key on prime, try turning key off. Then do key on prime again. Do this several times. If pressure builds up to normal 43 psi after several key on primes, pump and/or filter is bad. There's also a filter sock on pick up assembly in tank. It could be clogged also.

I'm not familiar enough with the b-body to know if tank can be dropped without dropping rear axle. F-bodies need to have rear axle dropped to get tank out. Many people who own f-bodies will cut a trap door in sheet metal above tank to make pump change easier.

On my 90 camaro, I can change the fuel pump in an hour, on the side of the road. I always carry a spare. I'm sure you could cut a trap door in roadmaster, but I don't know where you'd have to cut.
 

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Discussion Starter #15
I'll do some more leak down testing on the fuel pressure before moving on -- but I've got to get the car to repeat the problem while the fuel pressure gauge is attached. I'm leaning towards this intermittent no-start issue to be not fuel related, because my experience with clogged fuel filters and old pumps is that they show themselves at high speeds and with the car not wanting to go at high engine rpm...but I'm not ready to say that the fuel pump (or its wiring, or the fpr) wasn't the problem. I hate intermittent problems. I'd rather this thing were just dead in the water, it'd be easier to solve!! Thanks for the all the help -- and i found a video on youtube that shows fuel pump replacement with Roadmaster, you can drop the tank without dropping the axle, but you do need to drop the tank to replace the fuel pump.

I'll have more time to do debugging later this week.
 

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Discussion Starter #16 (Edited)
Success! Or at least I think I've solved the problem. I couldn't get the car to not start in my driveway, so put my fuel press gauge and laptop and drove on an errand. Of course the car decided to not start in a Vons parking lot on my way home. Great, now I can fix it. Put the laptop in, recorded some data, all of the parameters looked good. Put on the fuel pressure gauge, and presto- 3-4 psi. Enough to get the car to sputter to life, but not enough to run.

Tried multiple times, same result, no fuel pressure, no running.

Here is the best part -- I called AAA and asked for a tow home (it was only 2 miles) and when the flatbed arrived, the driver said, "I can get you running enough to run home." I said..."okay." He pulled out a large hammer and hammered on the fuel tank (which made me cringe because it is a plastic tank..yikes) and instructed me to try and start the car as he continued to hammer near the fuel pump. Yup, started right up and drove it home. Now time for a new fuel pump. Delphi or OEM GM?
 

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Either one is a good choice. Just make sure you buy it from a business that's on "the up & up". Too many China conterfeits being sold as the real deal. One sure way to know if it's for real is the price. If price is way low, you can be pretty sure it's a counterfeit pump you'd be buying.
 
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