91 2.8 only starts on full throttle - S-10 Forum
 
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post #1 of 30 Old 08-25-2014, 11:12 AM Thread Starter
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91 2.8 only starts on full throttle

I am in desperate need of some direction!

First I'm sorry this so long but I wanted to give as much background as I could to help you guys help me. I have just rebuilt my 2.8, I have a laundry list of new parts on it including sensors. First startup on new engine was good even better after setting timing exact at 10 degrees BTDC. Drove it up and down the road ran good no engine noises at all. Only thing that was an issue was a hard cut-out in middle of second gear under acceleration and a slight stumbling in higher gears. On further review (pulling codes) it said I had a low voltage on tps sensor, so after some probing and adjusting I got that fixed. It seemed to help but still had the same symptoms.
One night after work it started up and ran fine for about 5 miles then upon accelerating from stop sign the truck started to buck like crazy major loss of power when getting of clutch and accelerating. It was bucking so violently I had to shut down truck. When I got it home I found that my distributor had turned a bit due to loose bolt, so I reset timing and seemed to fix the bucking but still had stumbling and cut out during acceleration. But I could still drive as long as I didn't get on the accelerator too hard. Drove it for 4 days 20 miles each way with an 8 hour break between just with stumbling problem.
I had an appointment early on day before work and truck drove awesome. After my appointment I came out and truck started but once I put it in gear to move it died and then restarted engine was surging. Truck would idle fine but once you hit gas it died. So a friend of mine com got me and noticed the injectors were not really spraying that good had some drops of fuel along with a light spray. After letting the truck sit for a few hours I went back to check it out, the truck started right up where I could move it to a parking lot.
Once getting the truck home I bought a new fuel pump and filter and seemed to fix the spray of the injectors. Now my question is why will truck not start without full throttle? I have never had to use throttle to start before. Now if I take my foot off it will die and idles like crap. Please Help!

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post #2 of 30 Old 08-25-2014, 11:45 AM Thread Starter
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Re: 91 2.8 only starts on full throttle

Parts that are new since installing engine are as follows:
oil sending unit, tps sensor, IAC valve, Temperature switch, thermostat, distributor cap and rotor, spark plugs and wires, fuel pump, fuel filter, MAP sensor. I have also started the truck and did a light tap test to ECM while truck was running there was no change in idle. Also did an OHMs reading on temp switch it is currently 90 degrees and I got a reading of 1507ohms on sensor. Seems engine has a bit of a lope to it when idling.
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post #3 of 30 Old 08-26-2014, 11:18 AM Thread Starter
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Re: 91 2.8 only starts on full throttle

Just had ignition module tested. Tested 10 times passed all.
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post #4 of 30 Old 08-26-2014, 11:54 AM Thread Starter
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Re: 91 2.8 only starts on full throttle

Pulled a plug and cranked engine I am getting good spark, plugs are soaked with fuel though.
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post #5 of 30 Old 08-29-2014, 03:30 PM Thread Starter
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Re: 91 2.8 only starts on full throttle

Today I reinstalled truck bed, hooked up all grounds for tail lights thinking maybe that had something to do with it. Turns out it didn't. I also tried unhooking fuel injectors and cranking engine, truck would start but quickly die do to no fuel from injectors. Hook them back up and nothing just keeps flooding plugs with fuel. I am going to replace coolant temp switch tomorrow, even though it is a new sensor that is in it now.
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post #6 of 30 Old 08-31-2014, 12:32 AM
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Re: 91 2.8 only starts on full throttle

Here are a couple of things to try. Check ground wire between battery and block. On the back of drivers head(up against firewall) are a few important grounds. The gas tank sending unit has two grounds that come together and are screwed to frame. On mine they looked fine but two fuel pumps later I determined that the copper on inside of wire insulation had broken leaving me with an intermittent stalling issue. If your check engine light fast blinks with ignition on you have a prom issue with ecm. Check fuel pump relay( your ecm energizes fuel pump relay for 10 seconds upon start up, after that, as long as your oil pressure sending unit sees at least 4 psi it keeps fuel pump energized. Verify spark plug wires on correctly(yes, I know thats crazy for me to ask) and last but not least remove your serpentine belt and try to start it but don't leave it running more than 30 seconds. I knew someone who's a/c clutch froze/locked up and the engine would not start because of the amount of drag. Good Luck!
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post #7 of 30 Old 08-31-2014, 12:43 PM Thread Starter
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Re: 91 2.8 only starts on full throttle

Thanks for reply. I thought I had everyone stumped for a while. I will try and get back with you. Thanks again.
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post #8 of 30 Old 08-31-2014, 01:15 PM Thread Starter
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Re: 91 2.8 only starts on full throttle

Ok. I checked grounds, took off serpentine belt, pretty sure relay is working right. When I turn key fuel pump runs for 5-10 seconds even hear it when cranking. When I turn key to on position before starting check engine light flashes once and then stays on.
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post #9 of 30 Old 09-05-2014, 04:22 PM Thread Starter
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Re: 91 2.8 only starts on full throttle

I got another computer for truck today inserted my PROM into it and no change. Truck is still cranking and pouring fuel to it, but did start with accelerator on floor. As I let off of throttle truck was popping and sputtering then eventually died when i completely got off of throttle. I have had some friends telling me it could be the wrong pump (too Much pressure) possibly? Even maybe bad fuel pressure regulator? Any ideas!!!
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post #10 of 30 Old 09-09-2014, 11:23 AM Thread Starter
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Re: 91 2.8 only starts on full throttle

Rebuilt TBI today. Truck still only starts on full throttle but now after a bit of warming up I can get it to idle without apply throttle. Sounds a bit rough and blows black smoke.
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post #11 of 30 Old 09-09-2014, 12:58 PM
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Re: 91 2.8 only starts on full throttle

Just for chits and giggles, I would replace the coil. not that expensive. I had somewhat the same symptoms,but with a carburated motor. would have to floor it to start it and the plugs became fuel fouled, black smoke. I had changed to a performance intake on a V8 once and the car started doing it. Thought the motor couldnt handle the intake and holly carb. changed the Jets a few times still didnt fix it. So I put the stock manifold and carb back on. The problem was still there, I was like WTF. Then a friend told me to check the Coil. I changed it and all was well even stuck the performance stuff back on. So at the same time I did the swap, the coil went bad. Murphys law.
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post #12 of 30 Old 09-09-2014, 01:39 PM
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Re: 91 2.8 only starts on full throttle

Have you put a gauge on the fuel line to check the pressure? Could be the pump pumping too much fuel since your plugs are wet. You've done your homework here, but I guarantee you that it's more than likely something really insignificant that you're overlooking. I spent three days trying to get my truck to start after the EFI conversion, and it was because the ignition wire had slipped off the breakaway connector I had installed on it....



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post #13 of 30 Old 09-09-2014, 04:13 PM Thread Starter
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Re: 91 2.8 only starts on full throttle

I will try these things and get back with you guys. I have an extra coil I can change out. I called the manufacturer of the fuel pump. His exact words were fuel pumps don't pump by pressure they pump by volume. The fuel regulator provides pressure to engine by receiving volume from pump. I am also going to check compression on cylinders to make sure my rockers are adjusted to tight maybe a valve or valves are staying open. Friend told me maybe after running for a while everything got pumped up with oil and maybe too tight. Thanks for help.
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post #14 of 30 Old 09-10-2014, 05:33 AM
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Re: 91 2.8 only starts on full throttle

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Originally Posted by ross.brown1 View Post
I will try these things and get back with you guys. I have an extra coil I can change out. I called the manufacturer of the fuel pump. His exact words were fuel pumps don't pump by pressure they pump by volume. The fuel regulator provides pressure to engine by receiving volume from pump. I am also going to check compression on cylinders to make sure my rockers are adjusted to tight maybe a valve or valves are staying open. Friend told me maybe after running for a while everything got pumped up with oil and maybe too tight. Thanks for help.
I know you said the plugs are new, but even new plugs will get fuel fouled after a couple times. So even though you are drying them out, they might already be fuel fouled and cannot throw the spark needed anymore. Might be best to start with a fresh set.
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post #15 of 30 Old 09-11-2014, 05:35 PM Thread Starter
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Re: 91 2.8 only starts on full throttle

I thought of that also. Im going to get a set tomorrow. I will follow up Saturday. Thanks for help.
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post #16 of 30 Old 09-13-2014, 10:52 AM Thread Starter
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Re: 91 2.8 only starts on full throttle

I changed plugs just a little bit ago. Truck fired right up but still has black smoke and idle is still little rough.
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post #17 of 30 Old 09-13-2014, 10:19 PM
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Re: 91 2.8 only starts on full throttle

Vacuum leak some where? A vacuum leak can cause hard starts and poor idle and or die. Have had that happened before.

correct me if I am wrong, the o2 sensors may see the lean condition and cause the comp to dump more fuel due to the extra air not seen by the maf/map sensors.
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post #18 of 30 Old 09-14-2014, 07:13 AM
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Re: 91 2.8 only starts on full throttle

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I changed plugs just a little bit ago. Truck fired right up but still has black smoke and idle is still little rough.

Ok, now you have fresh plugs, do not run the engine for long periods till you find the problem or you will foul the new ones out. Have you switched the coil yet ?
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post #19 of 30 Old 09-14-2014, 06:12 PM Thread Starter
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Re: 91 2.8 only starts on full throttle

I am changing coil tomorrow. I believe my problem lies with my valves. I put a compression gauge on #5 cylinder today and cranked engine and had 0 compression. I have already checked rocker arm and valve lash on drivers side. Tomorrow I will adjust passengers side valves and change coil. Truck starts easy just idle like crap and black smoke puffs.
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post #20 of 30 Old 09-15-2014, 04:53 AM
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Re: 91 2.8 only starts on full throttle

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I am changing coil tomorrow. I believe my problem lies with my valves. I put a compression gauge on #5 cylinder today and cranked engine and had 0 compression. I have already checked rocker arm and valve lash on drivers side. Tomorrow I will adjust passengers side valves and change coil. Truck starts easy just idle like crap and black smoke puffs.
Yeah, if the valves are no adjusted correctly you will have issues like that. Adjust make make sure they are correct. I would think though all of them would have to be out of adjustment for all of the plugs to be fuel soaked, or was it just a couple of the plugs that were that way?
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post #21 of 30 Old 09-15-2014, 11:28 AM Thread Starter
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Re: 91 2.8 only starts on full throttle

A mechanic buddy of mine seems to think even though I preloaded lifters with oil after a week of running engine they may have loaded completely with oil making them too tight causing my issues. When I checked drivers side they didnt seem to tight to me but I readjusted them anyhow.
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post #22 of 30 Old 09-15-2014, 04:37 PM Thread Starter
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Re: 91 2.8 only starts on full throttle

Adjusted all valves. Truck starts right up, hardly any black smoke. Even idle is lots better. Will attempt testdrive tomorrow . Fingers crossed.
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post #23 of 30 Old 09-16-2014, 02:19 PM
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Re: 91 2.8 only starts on full throttle

Looks like you found your problem.

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post #24 of 30 Old 09-22-2014, 12:10 PM Thread Starter
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Re: 91 2.8 only starts on full throttle

After driving, yes I believe I found my problem for the most part. Truck runs exceptional except for still running just a bit rich. When idling I don't have black smoke but I still have a raw fuel smell. At night on cold start ups it seems rich for about 10 minutes and then better. I downshift few minutes after start up on way home and noticed a little backfire and a bit of a flame out tailpipe. Like I said though about 5 minutes later it doesn't seem as rich and no backfires. ??
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post #25 of 30 Old 09-23-2014, 08:18 PM
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Re: 91 2.8 only starts on full throttle

Just for shits and giggles if you have tbi reach around back and feel if the regulator is leaking fuel.

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post #26 of 30 Old 09-26-2014, 07:04 PM
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Re: 91 2.8 only starts on full throttle

black smoke - rich or poor burn(wrong timing or bad fuel mixture)
backfires, timing.

you stated something about the distributor was loose and had to retighten.

this could potentially cause your problems.

just a thought

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post #27 of 30 Old 09-30-2014, 06:36 PM Thread Starter
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Re: 91 2.8 only starts on full throttle

I am not seeing any fuel leaking anywhere. The smell I get is out of exhaust (rich fuel smell). Checked timing it is dead on.
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post #28 of 30 Old 10-02-2014, 08:30 AM
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Re: 91 2.8 only starts on full throttle

Have you tried another coil yet ?
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post #29 of 30 Old 10-02-2014, 09:00 AM
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Re: 91 2.8 only starts on full throttle

Did you disconnect the bypass wire for timing before setting?

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post #30 of 30 Old 10-02-2014, 02:18 PM Thread Starter
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Re: 91 2.8 only starts on full throttle

yes disconnected wire on passenger floorboard.
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